What would he have to gain by lying about spending even more customer money? I see it as a boast of how well he was playing the game; and also an admission that he was trying to buy influence.

If he primarily donated to one party, the donations advance Sam's political views.

If he equally donated to both parties, the donations advance FTX's interests.

I believe that Sam would advance his own interests then lie to make himself appear neutral.

Yes I concede this is possible too.