Is salvation by faith alone, or by belief that salvation is by faith alone? It's the faith that saves, not the belief that faith is what saves. The object of faith is Christ, not one belief or another. If someone has faith in Christ, but doesn't know that salvation is by faith alone, he's still saved, since he has faith. Satan probably knows that every Christian doctrine is true, but that obviously doesn't make him a Christian, since he lacks the necessary faith.

The papal bull Unam Sanctam states:

We declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff.
We say that's false, because salvation is by faith alone, not by being subject to the Pope's political power.

Now, since salvation is by faith alone, when someone says that salvation isn't by faith alone, but also by something else, it doesn't matter what that other thing is; just as belief that Catholicism is true isn't necessary for salvation, likewise it's not necessary for salvation to believe that Catholicism is false. The only thing that matters is faith.

I like how the Anglican Articles of Religion put it:

XVIII. Of obtaining eternal Salvation only by the Name of Christ.

They also are to be had accursed that presume to say, That every man shall be saved by the Law or Sect which he professeth, so that he be diligent to frame his life according to that Law, and the light of Nature. For Holy Scripture doth set out unto us only the Name of Jesus Christ, whereby men must be saved.
Some members of the Roman Catholic Church have faith in Christ, some members don't. Not all members of the RCC are Christians, but it's perfectly possible to be a Christian and belong to the RCC at the same time. So whether a member of the RCC is a Christian or not depends on that individual member's faith, same for members of any other church/sect, Protestant or otherwise.