The Leonine Sacramentary is an ancient text. It was not promulgated under Leo XIII. It would be nice to have the quote from it. It may or may not be available online. I remember finding, if not the whole, some of the text in either English or Latin of the Gregorian Sacramentary in trying to find out about the way the Holy Week or simply Holy Saturday was carried out I think.

The oldest source of liturgical texts of the Roman Rite is a manuscript in the library of the cathedral chapter of Verona in northern Italy (Cod. Bibl. Capit. Veron. 80), commonly known as “the Leonine Sacramentary,” a collection of these libelli Missarum originally made in Rome itself. Its dating and raison d’etre have been the subject of a huge amount deal of scholarly debate; the 1966 critical edition by Dom Leo Mohlberg OSB includes a bibliography on just the question of the dating, with over 80 entries. Broadly speaking, the Verona manuscript seems to be a copy made in the first quarter of the 7th century of a collection made about 40 years earlier. The name “Leonine Sacramentary” is essentially a fancy of its discoverer, a canon of Verona named Giuseppe Bianchini (1704-64), who was in his time a highly respected scholar of Christian antiquity.

The collection is in every way extremely irregular, as are many of the individual Masses it contains, several of which have multiple alternative collects, or two prefaces, while others are lacking various parts. The first three quires of the manuscript are now missing, and so if it ever had a prologue which explained why it was made, and made as such, with less rhyme or reason than one would expect as to both the contents and their arrangement, this is now lost. However, there is a conjecture which I think would well account for its wild irregularity.

For almost 20 years in the mid-6th century, the Italian peninsula was wracked by a terrible war between the Ostrogoths, who had ruled Rome and most of Italy since 493, and the Byzantines under the Emperor Justinian, who sought to regain control of their ancient capital and the heart of the Roman Empire. Beginning in March of 537, the First Rome was besieged for a year, and most of its famous aqueducts were broken. In 546, the city was sacked, and in 549-50, subjected to another siege, at the end of which, a notable portion of the population fled. It is guessed that about fifty years later, when St Gregory the Great was, as Pope, effectively the ruler of Rome and environs under the suzerainty of Byzantium, the population was down to perhaps around 80,000, perhaps rather fewer than that, living in a city built for 1.5 million.

Evidently, you did not understand the point Davies was making:

He makes no reference to Leo XIII (nor does the Leonine Sacramentary have anything to do with Leo XIII.

The Leonine Sacramentary (which has nothing to do with any of the Leos) was in use in the 4-7th centuries.

He's saying that a book in the 1890's said that the form was unchanged since the Leonine Sacramentary, but that the author hadn't noticed that the "ut" had crept in.

Pius XII stated very clearly what "the form" of the Sacrament had to be after 1947. And in his required formula, "ut" was present.

The "Leonine Sacramentary" is not a perfect text. Do you see all the footnootes at the bottom of the pages? Do you see how some of those notes reference alternative readings of the actual words in the text?

These ancient docuмents are sometimes nothing but fragments with parts of pages missing. The scholars have to make assumptions in some cases. That is one of the reasons for the footnotes: to let other scholars know that the original text was ambiguous. Read the Introduction to understand how uncertain it all is. It is not to be taken as "authoritative." It is a mess, as the author explains.

As I said, the current, authentic "form" of Priestly Holy Orders is the exact "Preface" to be found in the Pontificale Romanum used by Pius XII. The "Leonine Sacramentary" is of historical interest only.

It certainly settles the matter because all that's needed is positive doubt.  Nobody's saying we can prove the Rite is invalid, as the authority of the Church would be required to determine that with certainty.  But when the essential form has been CHANGED by a completely unreliable and questionable "authority", that suffices to establish positive doubt.  Given the positive doubt criterion, where that suffices to treat the Sacrament as invalid in the practical order, the burden of proof is 100% on the side of those who assert that it's valid.  I know that I'm not going to put my soul at risk by going to Confession to NO-ordained priests.  But if Sean Johnson or others want to believe that it's certainly valid, well, that's their choice, and more power to them.  But, you know what, I'll bet that Johnson would never put his money where his mouth is and start going to Confession to Novus Ordo priests.

But let's extend your example to make it more apropos to the NO Rite.

1) I bought rice for us to eat.
2) I bought rice.  Let's eat.

In #1 we're clearly eating the rice.  In #2, you bought rice (just got back from the store), but we're going to eat spaghetti now.  Rice is for tomorrow.  And the #2 is basically saying, "I'm back from the store now (having bought rice for tomorrow), so, since I'm back, we can eat our spaghetti dinner."  Or I'm back from buying rice in an attempt to save my wet cell phone, so now we can eat our steak dinner.

Does #2 imply that you're going to eat the rice?  Maybe.  Maybe not.  But it's not certain.  And it's not unequivocally certain whether we're going to now eat the rice that I just bought.

This is what all self-professed Traditional Catholics should at least think about any New Rite.  But that might mean one cannot go to one's nearest FSSP, ICKSP or even SSPX mass.  Or believe their favorite NO-ordained priest on the internet is not truly a priest.