In this particular instance, it does seem that the question was closed mainly due to its being answered on another site, and "off-topic" was just the least inapplicable of the official "close reasons".
However, it's not obvious to me that questions that are purely about some mathematical foundation should be on-topic here just because the foundation has been used in a proof assistant. If there is nothing at all about the question that relates to the proof assistant, but it is purely a mathematical question, wouldn't it be better to ask it on Math.SE or MathOverflow?
Dependent type theory is strongly associated with proof assistants in many people's minds, but there's no intrinsic reason for this. Some proof assistants use ZFC; are all questions about ZFC therefore also on-topic here? I wouldn't think so.