While you can disagree with Matt's approach, it actually feels like this dispute is more between WP Engine and the WordPress project than it is between WP Engine and Automattic. WP Engine not contributing to the project hurts Automattic a little, but the largest, most profitable companies in the WordPress ecosystem not contributing to the project are an existential threat to the sustainability of the Open Source project.
Companies will always optimize for profits, and contributing to an Open-Source project is only profitable when you are in it for the long run. And we all know that PE firms (which play an important role in our economy) are not in the game for long-term gains. Silver Lake is doing what they are meant to do — maximize profits in the short-term so that they can turn around and sell WP Engine for as much as possible.
Matt is using the leverage he has to ensure Silver Lake is forced to do something that is good for the WordPress project but will never happen because it cuts into WP Engine's P&L.