It's just a guess. There are 40,000,000 men worldwide in the top 1% of any metric, and I am fairly certain it's safe to assume that Mahomes can throw, lift, sprint, run long-distance, and stands tall enough to place in that top 40M every time.

I suppose some argument could be made that he's probably more in the 90s for everything rather than 99, but he's a full-time professionally trained athlete in the most dominant NFL team currently active, so I don't think calling 99% absent any formal data is too crazy.