A surety bond is basically a loan, through a bail bondsman. The bail bondsman has insurance in case you don't show up.
A property bond is where the court gets a lien on your property. It sounds like this is what happened.
(Nothing against Stanford or professors, but academia pay peanuts and professors are usually very far from having a net worth of 9-10 figures, faaaaaaaar awaaaaaay)
Obviously not relevant in the context of $250 million.
I would guess that their net worth was north of $10M just based on those earnings, subsequent investment, and their house (which is apparently worth $3M+).
Of course, SBF has had some incredible arbitrage wins over the years, and he presumably offered his parents the opportunity to run a million or two through his money making machine. If they were more conservative with their gains that he was, they could easily have $20 or $50 million. I agree it's unlikely that they have 10 figures, though.
Sureties don't have to be bail agents / bondsmen.