I can understand a make work decision like the FC-31, which isn’t beyond how Chinese SOEs operate, but a missile is an entirely different matter. You don’t just make a missile whose primary reason for existing is to be able to fit 6 into the J-20’s weapons bay with an off hand shrug. There are systems integration factors to consider. You’d need to coordinate with the J-20’s development to make sure that it can mount 6 of those missiles, or else that completely defeats the point of the missile, and features like how many mounts the weapons bay can hold are strictly dictated by the PLA. In order for the J-20 to mount 6 PL-12s, the J-20 needed to be able to have 6 mounts in its weapons bay, and if the J-20 was developed with 6 mounts in mind that had to have been something decided by the PLA early on.
AVIC wants six PL-15s for the FC-31 anyway even if the PLA didn't ask for it for the J-20, and we don't know if the PLA asked for it. Pb didn't elaborate on why the PLA isn't insisting on it, only that there's a possibility the PL-15 folding-fin version mightn't enter service.
Also, the PLA doesn't control the design parameters of their equipment, only the functional requirements. The institutes, all owned by AVIC, decide the design parameters. Things like fire-control system architecture and design of internal pylons are out of the PLA's control. Whether the list of functional requirements include six PL-15s is ambiguous as the PL-15 is a class above the AIM-120 and the PLA's original goal for the J-20 was to match the F-22, later changed to matching the F-35, and neither Fs have a PL-15-type missile, only the PL-12-type AIM-120. The likely functional requirement issued by the PLA was six PL-12s, matching the F-22's six AIM-120s. The PL-15 came afterwards.