You don't have to know everything. It's not, in fact, possible to know everything in accessibility. Every principle, every standard, every law, every semantic element and their roles, every disability and their impact on the web.
So there's no point in acting like you do when someone asks you a question you don't know the answer to. You're much better off saying:
I don't know. How do you do that?
And then be actively engaged in an honest conversation about solving the problem in front of you. I much prefer this attitude of "I don't know, but let's figure this out."
Because it's in the "let's figure this out" mindset that growth and innovation happens and problems are solved.