I don't intend to assert that PhDs don't do a lot more work that 20 hours. I know first hand they do. The work they typically are compensated for (TA/RA/FA) are typically capped at 20 hours a week. This also aligns with the legal limits for international students on F1 visas.
I suspect that even the people who wrote that rule knew it wouldn't actually mean anything practically. It's just a convenient excuse to exploit someone in a vulnerable situation.
