(are there Americans who use the word "tart?" it isn't in my spoken vocabulary)

Canadian, if it can count, and I do use it, though possibly it is because my life is overrun with English people. I wouldn't say I use it often, but thinking about it, I'm more likely do so when referring to an English person (no matter whom I'm talking to). So I don't know that I take it all that seriously as a word, more of a joke, really, or a word association, which is how I see Scully using it here, I suppose.

All of which is to say I bought it, it didn't pull me out of the story.