CathInfo.com

Now, among those things which the Church has always preached and will never cease to preach there is also contained that infallible statement by which we are taught that there is no salvation outside the Church.

However, this dogma must be understood in the sense in which the Church itself understands it. For Our Saviour gave the things that are contained in the deposit of faith to be explained by the ecclesiastical magisterium and not by private judgments.

Ok. But his being alive when published is relevant to the letter being a "fraud."
The HOL is very bad news in context especially, as is the whole "Boston Heresy" affair, that's certain.

This letter, known as the Suprema haec sacra, from the first three words of the Latin text, is of unique importance for the study of this section of sacred theology. It is an instruction of the Holy Office, sent out with the approval and at the bidding of the Sovereign Pontiff himself. As such, it is an authoritative, though obviously not an infallible, docuмent. That is to say, the teachings contained in the Suprema haec sacra are not to be accepted as infallibly true on the authority of this particular docuмent. Nevertheless, the fact remains that much of its teaching - indeed, what we may call the substance of its teaching - is material which has appeared in previous docuмents emanating from the Sovereign Pontiff himself and from Oecuмenical Councils of the Catholic Church. Fenton