I've never heard your rule, and I don't think it's very helpful as it stands.
One use of could is to make polite requests, eg
Could you bring my bag up for me?
The answer is most often not can or could but will:
Yes, I will!.
No, I won't, because I'm not going that way.
Sometimes the answer is can, usually meaning that the person is confirming that they are able (or unable) to fulfil the request, but for some reason they are not going to do so right now:
Yes, I can; I'll bring it later.
No, I can't, because I'm not going that way. (Slightly politer than won't here)
Could can also be used, indicating that there is some problem or complication that must be dealt with before they can fulfil the request:
I could bring it, but it might make more sense for you to ask Joe.
Your example, though, is a quite different meaning of could: it is not making a request, it is asking a question, and could is functioning simply as the past of can. In this case, it makes perfect sense for the answer to use could.