You are right, fusion reactors are used.
They overheat so hard that their fucking ammo cooks off even if it is stored in the extremities, i do not want to stand anywhere near a diesel-powered Mech, or even worse, pilot one when it fires a PPC Salvo.
tbh, if they were smart and knew what they were doing, they should have done it that way from the beginning, but they want to cut corners. AI is great at rapid prototyping; but it’s not trustworthy enough to call that a finished product.