I think most CMs are Disney fans so will have a fair bit of knowledge; they're certainly not working there for the fantastic pay or benefits! Although there are always some exceptions - one of my sister's roommates has gone to Magic Kingdom for Traditions (their orientation) and to watch Wishes once before going home straight after and that is all the park time she's had in the 1.5 months they've been there! She's an odd one though, to say the least!
Before starting work all CMs are given a presentation that includes a brief history of the company, but I believe focuses more on company values, etc. Some may learn more trivia, etc. as part of training for their role. For example, character performers will know everything there is to know about their 'friends'! That may be the case with bus drivers; I don't know. I also don't know if Disney would suggest a CM brush up on their knowledge of Disney movies if it were affecting their job, e.g. if they were working at BOG without having seen Beauty and the Beast so were unable to keep character with the guests.
I can definitely see that Universal would want to make sure that all their employees in WWOHP are very familiar with the Harry Potter universe; it's far more detailed than any Disney universe (simply by being 7 books/8 movies and designed for an older audience) and I think the fans would be pickier about details too!