probably discussed in the bitcointalk thread, but how do we know it's not just the creator of the puzzle reclaiming his own bitcoin?
By what means might you prove or disprove this theory?
You'd just need to download the 6,505,548 TB list of keys and re-derive the public key for each to check that they're valid; unfortunately it would take in the ballpark of a kiloyear of compute time assuming you have 3x RTX 3090s.