No one is an expert in everything, and Fellows don't expect you to be. What they need is to trust that when you speak with confidence, it's earned. Every time you bluff on a topic outside your expertise, you spend that trust.
“I don't know” is one of the most useful things a mentor can say. It saves the founder from acting on a guess, and it opens the next question: who does know? That's where your network and the nAItive mentor pool come in.
Fellows reach mentors for specific vertical expertise. Stay in your lane where it matters, be generous with general judgement where it helps, and be explicit about which one you're offering.