7.62x39 isn't actually as good at penetrating armor as .223, assuming the 2500 joules refer to high class American penetrators (5mm AP), it should actually be mostly immune to Chinese weapons.Imo they should ignore 7.62 that's what the Chinese used and they had to resort to building a rail gun.
The quote of 2,500 joules is not the cap but the bare minimum, the quote says OVER not UP TO.
In any case the power armor T-51B is designed primarily for laser resistance, indicating China had a large arsenal of laser weapons as well.