I'm with you on the snobbery of those that have seen the theatrical musical production.
My first recollection of it was the 1978 (Excellent) TV production with Anthony Perkins and Richard Jordan - and I'm quite certain not a single note was sung! I'm pretty sure there were several earlier non-sung productions.
Having said all of that - you could have knocked me down with a feather when I realised the story is actually based on a BOOK!
It is impossible for everyone to know everything about every subject, but because these people know Les Mis inside out there is the presumption that everyone else should do also.
I'm sure there are books, films, tv shows, plays that they haven't seen or don't know too much about.