I wonder how strong of a signal that is for a good engineer. There have always been people who've stood out as wanting to teach, wanting to mentor. Is it too much of my own bias to say that good engineers are good teachers?

I think that for example good designers are usually good teachers, so maybe.

Good design and good engineering explains itself; especially engineering that interfaces with other stuff. Well-designed things need less documentation.

Both, I think, require the same focus on clarity of intention as a good teacher.