Yeah, see, Bob is forward thinking and hella smart, so he knows that mixing ammonia and bleach creates chloramine, and Bob, who is a bit of a scientist himself (because those are not obsolete, and Bob is not obsolete, so he must be one; logic is also not obsolete), always has a bunch of hydrazine at the ready and just assumes that everyone else has, too. I mean, his floors are so immaculately clean that he must know a lot about cleaning agents.
In reality, of course, he just didn't know the specifics and just heard that mixing bleach and ammonia creates toxic gas. And since his mind is always on food, he could only think of mustard gas.