A member of our church, brought up Catholic, told me when she was a little girl in parochial school a nun told her the same thing about her non-Catholic father. This kind of thing is utter hubris either way (and cruel in the nun’s case).
I went to Catholic grade school from 1959 to 1967. Our teachers explicitly mocked Catholics who believed only Catholics go to heaven. One of them told us the old joke about the guy arriving in heaven and being told to be quiet while passing by the walled-in area containing the Catholics, “because they think they’re the only ones here.”
The Baltimore Catechism explicitly taught that people who are not visibly members of the Catholic Church can be saved.
I never met a Catholic who believed in Feeneyism until a cousin of mine became a Feeneyite as an adult.
I believe that many Catholics have been told that if THEY LEAVE THE CHURCH, they will go to hell because doing so is a mortal sin. Which is usually true. I believe they have misinterpreted this to mean that ALL who are outside the Church are damned, rather than what was meant, which is that LEAVING the Church is grave matter.