This is an extensive writeup on the subject. But the definition is not an opinion, the based on reified variety of socially constructed meanings to the word "Christian" discounting Catholics as Christians is only done by anti-Catholic bigots and people that ascribe to the religious practices of Catholicism without ascribing to the faith.
You may well have been told-differently by an anti-catholic bigot, as I was, but it was definitely to continue on anti-catholic bigotry within my Baptist church (I've heard much MUCH worse from the Pentecostals)
It is no more silly than the OP's request for "Muslims" to apologize ( many of whom discount each-other as "true" Muslims.) That said, the catholic church did fail to act against the nazi regime, something that it should apologize for. Further so did the protestant Lutheran church, a group that made up a significant portion of the population of germans that enabled Hitler.
It makes much more sense to request an apology from, at least, the german Luteran church, the Catholic church and any other church represented in Germany at the time that didn't speak out than it does to ask "Muslims" to apologize.
Muslims is not only wide, but it allows for any two Muslims to apologize to meet the criteria. Which, again, they have, to me personally (all while discounting the status as Muslim of the people that acted in 9/11)