I'm not convinced they couldn't. Depends what you mean by Einstein. You won't be formulating GR, but an IQ test could be doable.

At least if you have enough IQ to figure out how to solve IQ test problems on paper. Which shouldn't be that hard.

IQ tests are timed. Not everyone could be a slow Einstein, but perhaps you if you had 200-300 years might reach the same solutions Einstein did. If you choose to work on the same problems.

If you are a 5' tall basketball amateur, then even with 300 years of training you won't outplay the top NBA player.

It's different because to outplay the top NBA player you can't do it slowly. (You can compute slowly, though)

Maybe, the correlation isn't linear. Or Einstein USP wasn't just his IQ.