Presumably still has the shares and the paid tax bill on them, so hardly “out $300K” any more than they’d be “out $300K” if they bought $300K of $AAPL.
83b election is good advice, though, especially for very early stage (low strike price) companies.
I believe the mention of 83b election by the author is a red herring.
afaik 83b only applies to recently acquired stock, not options.
Many early stage companies will have early exercisable options (wherein you can exercise the options [paying the strike price] prior to the grant vesting, meaning you forfeit the shares if you leave before vesting). On those, you can do an early exercise and make an 83b election.