Law professors make much better money than most academics, with starting salaries more than double what humanities professors make. These are also senior professors who have been at Stanford for a long time, making good money and presumably investing along the way. SBF's dad is a famous tax law professor (he 'wrote the book' on federal income taxation that's used at law schools all over the country), so he understands finance/investing better than most academics.
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.