> We had an interviewee (is that a word?) today who said that he didn't know and to me that was a reason to hire him.

I've seen this in interviews too, but my experience on this response is a bit mixed. I appreciate the honesty, but saying you don't know is a dead-end to solving the problem. A better response would be "I don't know X, but it sounds similar to Y which I solved by ...". Even if X and Y are not closely related, it at least shows they have the ability to make these mental connections.

I'd say quite possibly so...if he also followed that up with "...but I could find out." (or something similar).

Saying "I don't know" is one thing - it's good to admit one's lack of knowledge.

Being able to then follow it up with the will to expand their knowledge - that is valuable IMHO.

Yes it was clear that he acquired his other knowledge through hard work and interest.

Maybe it is a word, but I think candidate is a more generally accepted term

Ah thank you, I know that word and now I have no idea why I made it difficult for myself :D

I've had recently that experience as well. (We hired him.)