That valid CSS unit measurement sounds like inane trivia to me. I've never had to use anything other than em, px, and % before and I've never even seen it in any open source projects.
Definitely. You'd be silly to asses the skills of a developer based on such a question. My point was more-so that companies should ask questions developers would know instead of asking questions most developers would probably Google the answers to.
I wouldn't reject someone because they don't know what rem is though! I'd think of the question as a bonus I guess.