I don't see that as being an important objection, because from the point of view of any single human… that's already true for the expertise found only in other humans.

I don't know the chemistry necessary to turn crude oil into any plastic, I don't know how to use a Lagrangian, I tried and failed to learn group theory (I've only got the basics, and I'm not confident about them), and I've still only got a toy model of special relativity (which is supposed to be the easier one) — and yet, not only are those all still useful, the four colour theorem is useful even though it's a non-surveyable proof: https://en.wikipedia.org/wiki/Non-surveyable_proof

Depends on whether we can instruct the AI to make something useful out of it. Today we have complex systems like CPUs that most humans don't understand.

Or an answer so simple: 42.

Abstractions are a thing. We use devices all the time that we don't understand entirely the workings of.