i agree to some degree, yeah. but ultimately, their weapons still need to be balanced as modular weapons. not just because Diable can use other factions' weapons, but also because other factions (and the player) can use Diable weapons. if their PD weapons are overpowered or underpowered, that's an issue for more than just Diable AI fleets.
plus, even if they prioritize using their own, all factions use other weapons as well. that's not something only the player would do. so even if the Diable PD choices would have bad flux efficiency, you'll still find a lot of PD weapons with good flux efficiency on Diable AI ships.
so: Diable PD weapons having poor flux efficiency as a weakness, balanced out by being strong in other areas, works quite well. each PD weapon can be balanced in itself to be overall worth using on many different ships, without being something you'd wanna use all the time.
but that doesn't really translate well into flux inefficient PD being a weakness inherent to the Diable faction. such a weakness would imply that they make up for that weakness by having stronger weapon or ship stats in another area (rather than those PD weapons themselves making up for their inherent weakness in their own stats), which would cause balance issues whenever you use Diable ships with other weapons, and whenever you use Diable weapons on other ships.
smoothing over the "problems" of a faction as a player should be about mixing for more varied combination and filling niches that a faction-exclusive loadout wouldn't. not about leaving out the weapons that were designed to be bad, and replacing them with weapons from another source that are just better as a result.