Wonder if Bannon did this on purpose.
Possibly. The stories are that he'll talk with Mueller and answer all questions, so it's possible that he let this "slip" now, if this is the most "damaging" thing he knows, rather than waiting and have this come out from a Mueller leak. I don't see how it amounts to anything - people in the WH would certainly talk about the Don Jr. story and how to respond to it. And the reason he was asked about it is because both the committee and Mueller have all the WH communications and emails, so they've already seen the "evidence" of "collusion."