It's likely in financial crimes that if you were able to profit further from your fraud, you need to pay that back too. Given how most assets and financial records can be tracked down, they saw what happened to the money. 2008 thru 2018 and even today has seen massive stock market gains. If he invested that $17 million and it grew, the investigators can probably see that. They can calculate his gains off of stolen assets and that's likely how they determined the penalties on top of the $17 million principal he must return.
Letting him steal $17 million and keep the gains but return the principal would be too good to be true, so that's likely where it comes from. And they probably already audited his finances to death to know what he can and cannot pay.
If you cannot pay it back, then you may declare bankruptcy likely and that's not a pretty picture in the US at least especially if you were at least a moderately successful employee at Apple.
Plot twist: He invested it all in Apple stock. </s>