The most recent pre-covid version of the US-China Air Services Agreement did not require each side to actually use 50% of the slots. Rather, the caps (i.e slot pairs that they could use) were equal.

China was ready to reinstate that agreement immediately after it sunsetted the Five Ones (just like it did with other bilaterals), but the US side didn't want this (it would have effectively been open skies because the 2019 caps were far greater than demand could support then, let alone in 2023).

The impasse was sort of broken last summer, but with much smaller caps that protect US carriers from the inevitable onslaught of competition from Chinese airlines.