isn't that what trademark law is for?

Trademark law exists to protect consumers from counterfeit goods. Basically, it exists to allow consumers to recognize the goods of someone they have established trust with. If I know I like Coca Cola, I can just buy any bottle that has the Coca Cola trademark and know what I'm getting, instead of worrying I'll get a completely different beverage.

This gets more complicated when multiple companies are selling services for the same open source software, because you want all of them to be able to say what their services are for, but you don't want one of them to tarnish the reputation of the software itself. But as long as all the companies are actually selling services for the same software, like in the case of WPEngine and the other company here, consumers are better served and informed if both call it WordPress.

No.