The Catechism of Pius X addresses baptism of desire in the section on baptism, not the section on the Creed.  Here's what it teaches:


16 Q. Is Baptism necessary to salvation?

A. Baptism is absolutely necessary to salvation, for our Lord has expressly said: "Unless a man be born again of water and the Holy Ghost, he cannot enter into the Kingdom of God."

17 Q. Can the absence of Baptism be supplied in any other way?

A. The absence of Baptism can be supplied by martyrdom, which is called Baptism of Blood, or by an act of perfect love of God, or of contrition, along with the desire, at least implicit, of Baptism, and this is called Baptism of Desire.

what exactly does implicit baptism of desire mean? 

As stated in the OP, The Catechism of Pius X was never written in English, it was in ltalian, so one can't just say "The Catechism of Pius X addresses baptism of desire in the section on baptism" and post something in English. Please tell us what year it was published and by whom? From the OP "There are two such Compendiums, one of which was edited by Fr. Kevane who updated it to reflect the teachings of VCII -

It is a slightly adapted translation taken from Msgr. Hagan's Compendium of Catechetical Instruction (1910)

https://archive.org/stream/CatechismOfSaintPopePiuxXTheSt.PiusX/Catechism+of+Saint+Pope+Piux+X%2C+The+-+St.+Pius+X_djvu.txt

Here's the description of the translation provided in the Foreword:

"After the Second Vatican Council, a number of new catechisms appeared which did not present Catholic 
Doctrine as it should be presented, and these new publications even included some very grave errors. 
Coupled with the new methods, whereby children are not required to memorize, two generations of 
children have grown up not knowing the Catholic Faith. For many years Rome did nothing. Now there has 
been published the new Catechism of the Catholic Church. It has been written for Bishops, who are 
required to adapt it to the needs of the faithful. One may fear that some Bishops will put off this task for a 
very long time, others will water-down the Doctrine even further, yet others will give only a partial 
presentation of the Doctrine, leaving important points untold. 

"Thus the need is still great for a Catechism to be put in the hands of the student in which he may find clear 
and complete answers to his questions. What better could be given him than the Catechism of St. Pius X, 
the holy Pope of the modern era? 

"To my knowledge, the Catechism of St. Pius X has never been published in English in its original text. 
There is one Catechism of Christian Doctrine, published by the Rev. Msgr. Eugene Kevane in Virginia, 
USA in 1974, but in fact, it contains a much later text which lacks much of the original text: it is the 
translation of the Catechismo della Dottrina Cristiana, the standard Italian Catechism, as it was in 1953. 
That Italian Catechism is in turn, a summary and reduction of the original Catechism of St. Pius X. The 
American edition in 1974 has further been "adapted according to the Second Vatican Council", thus losing 
much of the value of the original text (e.g. expressions like "Soldiers of Christ" are suppressed from the 
teaching on the effects of Confirmation). The only book where I was able to find the authentic text is the 
excellent Compendium of Catechetical Instruction by the Right Reverend Monsignor John Hagan, first 
published in Dublin in 1910, and containing for each chapter of the Catechism the relevant part from the 
Catechism of the Council of Trent , the questions and answers of the Catechism of Saint Pius X and Father 
Raineri's Catechetical Instructions, which were very popular in the nineteenth century. 

"We present here Msgr. Hagan's text with very slight modifications of style only. The current discipline of 
the Church on matters such as fasting has been included in smaller print to bring the text up-to-date without 
altering the original answers."

A theoretical example of justification by implicit baptism of desire would be the case in which a person 1) makes a supernatural act of faith, 2) combined with a perfect act of charity, and 3) the disposition of soul whereby the person wishes to conform his will to the Will of God.  Although the explicit desire for baptism is lacking, the implicit desire is contained within #3.  

Of course, we are dealing here with theoretical possibilities that are likely few and far between, but this is how theologians have always explained justification by implicit baptism of desire.  It is also important to note that many people mistakenly believe that justification by implicit baptism of desire only requires #3.  That is not the case.  Supernatural faith and perfect charity are absolutely necessary.  #3 simply supplies for an explicit desire for baptism.

...from whence it follows that the missionary is still essential, since a completely implicit act of supernatural faith is impossible (i.e., without knowledge and assent to some article of faith, said act would be merely natural, not supernatural), and would equate to Rahner's "anonymous Christianity," which in turn implies JPII's universal salvationism.

...from whence it follows that the missionary is still essential, since a completely implicit act of supernatural faith is impossible (i.e., without knowledge and assent to some article of faith, said act would be merely natural, not supernatural), and would equate to Rahner's "anonymous Christianity," which in turn implies JPII's universal salvationism.

Indeed, a supernatural act of faith does require belief in an object (article of faith ) that is not known to be true by the natural light of reason. Whether this proposition enters the mind by the preaching of a missionary, by reading a book, or via an actual grace is a another matter.

Yes, the "in the path of salvation" language is crucial and completely missed/ignored by the BoDers.  It's consistent with the teaching of Pius IX on the matter, that they are on a trajectory towards salvation and will, if they stay on that path, be given the necessary grace and enlightenment in order to be saved.  "In/on the path" means that they are on the way, but not there yet, that they have not arrived at their destination.  So this language was used deliberately.

St. Augustine and other great theologians applied St. John 10:16 to this very issue, emphasizing that Our Lord must bring the non-Catholics into the Church because they cannot be saved as they are.

And other sheep I have, that are not of this fold: them also I must bring, and they shall hear my voice, and there shall be one fold and one shepherd.

Notice how Our Lord speaks, indicating that it is He Himself who must bring these non-Catholics into the Church. The reason why the Church doesn't allow anyone to baptize himself with water is to teach us that we cannot merit the grace of joining the Church. We are to recognize that we are dependent on God to provide a minister for us. Even in cases of emergency, a person is not allowed to baptize himself; the Church would rather have a Jєω or a Muslim be the one who baptizes, provided he have the right intention, than permit a man to baptize himself.

It is a DOGMA of the faith that no man can merit the initial grace of justification. The apostles of "implicit baptism of desire" do not understand any of this. They write as if it is a matter of sheer effort for a non-Catholic to join the Church. It is very difficult, they say, because observing the natural law is hard for fallen man, but some can do it, and if they do observe it, then they will somehow baptize themselves implicitly, and become invisible members of the Church. The very reason God does not allow any man to baptize himself with water is to prevent this Pelagian nonsense.