It must’ve been a righteousness held by individuals and passed down through the genes. Or else it was taught by a few good men to a few good men, and held secret until it was safe to come out.
Of course, all of those righteous men were Catholics – unless those who think the real Church was in hiding was being kept alive by non-Christians.
So, generations of Catholics (presumably just a few good men) within the Catholic Church, kept the church hidden until one of them, Luther (a Catholic) decided to tell everyone that it was he was the one God had entrusted with teaching authority, not the Church.
Elsewise, they could be saying that there was no true Christian church on earth from Constantine (or before) until God’s new revelation to Luther (which had to be personal revelation if you accept the Bible as truth), which became accepted as equal to Biblical revelation and Sacred Tradition in the minds and hearts of his followers.
I’m not sure what they actually believe, but it all sounds like a sci/fi-Harry Potter cloak of invisibility theory–something remains but is unseen (Gate 9.5, was it?) . Or else it’s a Bermuda triangle-like situation in which it disappears from radar only to reappear more than a thousands years later, unable to account for the lost time.