For somebody deeply familiar with a topic and all the nuance around the problem and approaches that didn't work, I'd be quite surprised if they couldn't cajole it into explaining what finally did work in terms they understand. Unless it turned out that these proofs were just well beyond anything humans were anywhere near being able to figure out, in which case, no great loss that the AI figured it out because we were nowhere near it.
To humans? To check if the explanations actually make sense we would have to pause the new AI in while it's rediscovering the result and steer it with the explanation to see if it arrives at the result faster or slower/same.
It very well may be that explnations need to be further explained until they don't sound like gobbledygook to humans. If that's harder than arriving at the result then it's a sign of how far ahead AI actually is.