I'm not sure if it's true, but there were talks about WPEngine devs basically not being allocated any dev time on WordPress anymore (whereas Automattic devs spend tens of hours a week on the actual OSS project). That was one of the catalyst behind the whole drama where Matt called them out basically on stage during a conference talk. Which was just before (or at the same time) as he started talking about the royalty.
Now, again Matt's behavior has been super weird since he basically failed at articulating his position well, and his PR has been catastrophic.
But the fact is WPEngine still does not contribue to WordPress. That is their right, but everything matt has been doing is also "legal" and within his (and his company's) rights.