It was a bonus contract-read the article. These guys don’t work at will they have contracts...working at will is for the little people like us not the hotshots.
I may go fishing for the Feilbogen contract, just out of personal curiosity, but point out that the term you've selected, "bonus contract," will conflate various rationales for payment. I've been on the sending and receiving end of performance bonuses. Payment was contingent on meeting date targets or other objectively measurable performance targets, and also contingent on being an employee on the day the bonus was to be paid. DeSantis was working under what is called a retention bonus. From the tone of DeSantis' letter, had he known that the contract was worthless, he would have quit months ago.