How does that make anyone in the N.O. different from a Prot who thinks he is in the true Church?
Catholics who are not pertinacious heretics have the formal motive of faith, where they base their faith on the authority of the teaching Church, the Magisterium. Protestants do not. It has nothing to do with subjective sincerity.
Formal Motive refers not to WHAT I believe but WHY I believe it.
If I have the right WHY, i.e. because it's the Church teaching it, I am a Catholic, even if I am mistaken about the WHAT. I think the Church teaches [this error], and so I believe it FOR THAT REASON.
Prots not only do not believe the right doctrines, but even the ones they DO believe, they believe for the wrong reasons, without the right rule of faith.
That is why the Church teaches that if you deny one dogma, you deny them all. That's because you deny the authority behind ALL the dogmas, and whatever dogmas you still "believe", you don't believe them with the correct supernatural motive of faith but for your own reasons, making yourself (as St. Thomas points out) your own rule of faith.
In theory, you could have someone who believes every single dogma the Church has ever taught but simply have come to that conclusion due to their own reading of the Bible and their own interpretation. Despite MATERIALLY believing everything, using the term "believe" loosely, since it's not supernatural faith at that point, they do not have the right FORMAL MOTIVE of faith, and therefore no faith. No that this every happens in practice, but it's a hypothetical to illustrate the distinction.
It's the lack of such distinctions that causes many SVs to err, not distinguishing between the content of believe and the formal motive of faith, and failing to distinguish between the theological notes.