I don't know, I always liked that Joyce didn't need to figure out that Buffy's The Slayer and that Vampires Are Real in "School Hard" - she just sees that Buffy is resourceful, strong, takes control and looks after others. (Which, come to think of it, is another s7 parallel, even if it comes harder there.) And that's all she needs to know to have confidence in her. Her trust is in Buffy Summers, not in The Slayer.