Which benefits Mexican workers and by osmosis the US: the richer MX is, the more likely they'll become consumerist, and the US companies tend to sell a lot of consumerist goods.
Further, in the case of war, all those physical factories that are ostensibly owned by the Chinese could be nationalized by Mexico as they'd probably be more interested in being friendly with their immediate neighbour. Ditto for Canada: why is the US putting tariffs there?
Also: USMCA is only a few years old. There were already some Chinese-owned factories in Mexico at the time of its signing (which Trump at the time called "the best trade deal ever".) The Chinese factories didn't seem to have been a problem then.