It's possible he filed an 83b election and paid taxes at the value of the stock when he received it (even though it hadn't fully vested). If so, his tax liability would be significantly less.
In this case that is impossible. If you pick through Yahoo's filing you will see that what he was granted were "RSU"s not actual stock. This is something that Google uses too. Basically the value of an RSU is nominally one share, except that it is moderated by a performance evaluation. So if your moderator is 0 then your RSUs are 0 * n or 0 shares, if your moderator is 1.0 your RSUs are worth an equivalent number of shares, and if your moderator is higher than 1.0 your RSUs become a larger number of shares. The transform happens at the actual vest time. And fortunately there is a loop hole in the stock sales of insiders that allows for them to sell as many shares as they need to for tax liability purposes.
The reason the 83b is impossible then is that he doesn't know how many actual shares this will be. But that is ok since the company sells some of the granted shares to withhold for taxes.
This scheme of 'introducing a moderator variable' gives the company exceptional leverage over the employee as they can 'take back' all of their stock grant by setting the moderator to zero.
I don't know what his contract looked like but I'm guessing it has a minimum value exit (that was used to get him to change jobs) so it would have been hard for them to not pay him any of the equity.
