This Sedevacantist "X" account writes that some Sedevacantists misunderstand what is meant by "infallibly safe":
Source
If you read the text of Cardinal Franzelin, it doesn’t seem to say what this “X” fellow says it means. It seems to me that it supports our position. (See attachment)
The following is from Canon George Smith in an article in the Clergy Review expressing what Catholic’s have to believe:
“…that much of the authoritative teaching of the Church, whether in the form of Papal encyclicals, decisions, condemnations, replies from Roman Congregations -such as the Holy office - or from the Biblical Commission, is not an exercise of the infallible Magisterium. And here once again our cautious believer raises his voice:
“Must I believe it?”
The answer is implicit in the principles already established. We have seen that the source of the obligation to believe is not the infallibility of the Church but her divine commission to teach. Therefore, whether her teaching is guaranteed by infallibility or not, the Church is always the divinely appointed teacher and guardian of revealed truth, and consequently the supreme authority of the Church, even when it does not intervene to make an infallible and definitive decision on matters of faith or morals, has the right, in virtue of the divine commission, to command the obedient assent of the faithful.
In the absence of infallibility the assent thus demanded cannot be that of faith, whether Catholic or ecclesiastical; it will be an assent of a lower order, proportioned to its ground or motive. But whatever name be given to it - for the present we may call it belief - it is obligatory; obligatory not because the teaching is infallible - it is not - but because it is the teaching of the divinely appointed Church. It is the duty of the Church, as Franzelin has pointed out, not only to teach revealed doctrine but also to protect it, and therefore the Holy See “may prescribe as to be followed or proscribe as to be avoided theological opinions or opinions connected with theology, not only with the intention of infallibly deciding the truth by a definitive pronouncement, but also - without any such intention - merely for the purpose of safeguarding the security of Catholic doctrine.” If it is the duty of the Church, even though non-infallibly, to “prescribe or proscribe” doctrines to this end, then it is evidently also the duty of the faithful to accept them or reject them accordingly.”