frank, I'm sure that somewhere in this world during the entire 20th Century there was at least one sour, mean nun. They are human, after all. That hypothetical allowance made, the recovering Catholic school student shtick has been a well-worn badge of honor in Lib and Prog circles since at least the mid-1970s.
Remember that a lopsided proportion of primitives are lapsed Catholics, and that they tend to be the most bitter, angry, hate-filled primitives of all. Lapsed Presbyterians, lapsed Baptists, lapsed Unitarians, lapsed Jews, lapsed Buddhists, &c., &c., &c., aren't nothing in Hate when compared with lapsed Catholics.
I have no idea why this is, but there you have it.
And so when recollecting their experiences in Catholic schools, I think they make things up, just to vent their fury and hate.