I wouldn't consider it bizarre if a candidate has deep knowledge about something different than what I expected when I started. I'd consider that pretty normal actually, since most people don't work on the same thing.
I wouldn't consider it useless either, unless I'm hiring an engineer for a narrow focus on something (e.g. I want someone that knows BGP), but I've never done that.
> I've found that the ability to learn is fungible.
The ability answer a question effectively instead of exhaustively is important too, though, right?
Personally, I'd be more impressed by the candidate who answers this by first asking "is there a particular aspect or perspective of this task you're most interested in?", especially if they can highlight the broad aspects involved, and offer to drill down on any in particular.
I wouldn't consider it useless either, unless I'm hiring an engineer for a narrow focus on something (e.g. I want someone that knows BGP), but I've never done that.
I've found that the ability to learn is fungible.