And Will snickers behind his hand, AND FOR GOOD REASON. And Iskierka makes fun of him, too.

Yeah, I can definitely imagine William being able to handle cooking, only--he hasn't, up 'til that point, and I imagine he's vaguely mulish about learning up until the point that he realizes he's pretty good at it. And Tharkay shows him how to make all sorts of things Archie likes to eat at restaurants in addition to their boringass national meals like SOME KIND OF MEAT WITH STARCH.