Well, we all comment on how dumb she has become - but today was beyond belief. After all this time, it takes Peter asking her how she would react in 1968 if someone from the future came back, and comparing this to her situation, before she suddenly does a double take and "gets it" and says how "foolish" she has been!!!! Wowie!! It's actually dawning on her what the situation is?!!!!!
And then, he comments that she can't be expected to understand them. But, that's wrong. Aside from the fact that almost anyone going back would have some idea of what things were like back then, and try to fit in, in this case she's a teacher! Presumably history is one of the subjects she teaches David. She should have a pretty good idea of what it what.
All right, so her knowledge of history would cause her to be surprised to find belief in witches still existing at the end of the 18th century, but, having discovered that it does exist, her knowledge of history should also give her a clue about what it's like and suggest how she should react to it!!!!