>>All other WordPress-related businesses or projects can use the WordPress name and logo to refer to and explain their services
I think the policy is somewhat vague on this; does 'Wordpress Hosting' refer to and explain the offered service? Clearly. Is 'Wordpress Hosting' a "product" WP Engine is selling? Kind of, yah?
My understanding of trademark is also that "we've been doing this for ages and you didn't say anything" is a pretty solid defense, and "Wordpress Hosting" is about the most generic hosting service offered on the internet at this point, everyone and their dog offers it.