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I'm going to guess a couple of factors, just judging by the pictures on twitter:

* it's small but went through a lot of stuff, so it must have been going pretty damn fast

* it is metal but looks scorched on the outside. It looks like other parts you'd see at a museum that have gone through through reentry.

* it's too uniform of a shape to be a meteorite

A deorbit of 3T of ISS cargo is significant, and there was a decent amount of press in fairly mainstream news sites leading up to the re-entry. It doesn't seem that unlikely that he, or someone he knows, had heard about it and connected the dots.

He lives on Florida.

Notably home to the Kennedy Space Center, although that’s on the other side of the peninsula.