That's the amount of hours they contribute to the "Five for the Future" program which specifically refers to work on a subset of the WordPress project managed by Automattic. For example, WPEngine own and develop Advanced Custom Fields, a plugin used by hundreds of thousands of WordPress websites. The work they do on that isn't covered in the 40 hours. Likewise, WPEngine run the DE{CODE} conference but that isn't included in the 40 hours either.