Probably (the trademark equivalent of) fair use, because WordPress is what they are selling. If I have a basket of windows disks to sell, I can write Microsoft Windows on my price list because the thing I'm selling is called that.
This analogy came up recently when discussing Elasticsearch. It's flawed.
Free and open source software does not, and has never, required giving up trademark rights. I think the GPLv3 is even explicit about this.
In the Windows case it's fair use of the trademark because you're reselling something you previously bought. That's not applicable here.
WordPress is open source software, but a hosting service has a variety of characteristics unrelated to the nominal software. Besides, WP Engine are disabling key features of the product: of course that's misleading.
