That’s a niche viewpoint. Even then a lot of the time it’s rejecting Roman Catholicism as true. Or the common one “you can be a (Roman) Catholic but not a Christian” or at the most extreme (usually this person engages with strawmen) “There are Christians in the Roman Catholic Church, but they aren’t really Roman Catholics”
Protestants say that about Catholics, they are both still Christian
That’s a niche viewpoint. Even then a lot of the time it’s rejecting Roman Catholicism as true. Or the common one “you can be a (Roman) Catholic but not a Christian” or at the most extreme (usually this person engages with strawmen) “There are Christians in the Roman Catholic Church, but they aren’t really Roman Catholics”