I think pacu, unlike piranha, prefer eating fallen tropical fruit, seeds and nuts [hence the big grinding teeth in the back]... but that would not be an option in Oklahoma.
Evidently there was enough vegetative material in the lake to allow the fish to thrive. Those teeth look like they could do real damage to a swimmer but I’d still be more worried about a snapping turtle than this particular fish. Perch are bad enough for a male swimmer.