I can really understand why such great minds could have imposter syndrome. They put so much thoughts and work into their findings, and stumble upon so many obstacles, struggling for months or years to make it work, to finally reach a result that makes so much sense and eventually seems simple to them. In the end, they just think anyone with the same dedication, work and curiosity could have done it, and that they actually took quite some time to figure it out, and that the theory is actually imperfect and could be better, and the only reason they did it is because of some random hindsight anybody could have had...

What they forget is that their journey has only been possible by a combination of intuition, skills, dedication and raw intelligence that together form a genius other people just don't have. But they do have a point in the fact that people probably underestimate the amount of work and struggle necessary to reach their results. Imposter syndrome is when you think you put too time and work to realize how logic a series of deductions is, while most of those are actually clever and their accumulations put you light-years ahead of those wouldn't didn't make them, for whom it's just magic.

If you read his biography by Walter Issacson, he was considered fairly mediocre for many years in his youth and quite heavily internalized it.

Yeah I think it's the latter.