OP was not given all of the shares earned at the time decades ago and didn't realize that they should've been payed out, but after engaging in a lawsuit realized that the court would likely not grant the case give the statute of limitations.
Kinda like all the Sony game 'owners' not carefully reasing the legally binding contracts they're always signing realizing that they are not in fact purchasing a gauranteed lifetime access to the game.
Whether they "earned" them is disputable — the offer letter specifies one vesting schedule (25% every year), the "cover sheet" from the options agreement specifies other (25% every _quarter_).
So — the OP got the shares he was promised in the original offer letter; but later discovered that some of the documents he later got implied that the vesting schedule should be accelerated compared to what he agreed to.
I think that is probably the funniest way possible to earn a billion dollars; but whether he's "owed" that money/shares, is... up for a debate.
Now, 25 years later the author is preparing a litigation.