Startups run on short cycles. A reply that takes two weeks often arrives after the decision has been made. Being responsive doesn't mean being always available; it means founders can count on hearing back from you in a reasonable time, even if the answer is “not now”.
Follow-through matters even more. If you promised an introduction, a review or a call, do it. Every unkept promise teaches the founder that your words don't carry weight, and makes them hesitate before asking for help again.
It's completely fine to say no. A clear “I can't take this on right now” is far better than a yes that turns into silence. Tell the Fellow and the nAItive team, and we'll find someone else.