They're advertising the speed of their cache, which can easily be just a plugin, and the amount of servers, which well of course is external software that requires no changes to WordPress.
As for the endorsement part, I'll just copy what I said above:
At most, they just ambiguously used “Powered by WordPress Experts” once. I don’t see how the evidence misleads people into thinking there was an endorsement.
But yeah, the smart way out would’ve been adding a “WP Engine is not associated with WordPress.org”, at least one below the “WP ENGINE®, VELOCITIZE®, TORQUE®, EVERCACHE®, and the cog logo service marks are owned by WPEngine, Inc.” footer. All in the past now, though. At the best both companies are tomfools.
See, that's why I don't like talking about the same thing in multiple threads.