A couple of points:

1. No, we don't have proof that there was one person who thought it was worth 3x GDP. We have proof that there was one person who thought a 0.001% share of the company was worth 0.003% of GDP or whatever. They could think it was worth that much for plenty of reasons; maybe they thought the share price would grow for a bit more before collapsing so that they could make some profit, maybe they invested in order to just be an investor and have a say in investor meetings or however things worked back then. Maybe it was a status thing.

2. Why are we using the opinion of one random person to determine the value of a company

To add to this, this type of thing happens all the time in crypto.

A coin will release 1/1000000000th of it's eventual supply, have some trades at 10c and then claim the value of the entire supply as the headline value.

It's obviously dumb.