I think they didn't tell him about Lancelot being a shade because how would they explain knowing it? Cursed places and possesions, that can be found in books and surmised from evidence. But necromancy? Since the only way to detect is through magic? That would be pretty hard to explain, IMHO.

I'm VERY nervous about next week's ep. It looks very Gwen and Morgana centric which is not my cuppa :(