> My assumption is that they checked the proofs vigorously,

Perhaps with AI.

A manual human check of each one would take a few month at least. In peer review, there are horror stories in math about more than 1 year before the journal accept the paper. So 3 reviewers x 700 pdf = 2000 mathematicians, that is 10%-20% of the community according to an unreliable count printed by Gemini after scrapping r/math.

Also, in most cases the only people that can understand the proof in a so short time (let's say a few months!) is the small group of people working in similar problems, i.e. the same group of 20-100 guys/gals that you meet in every conference.