FYI: Matt Mackall is now Olivia Mackall [1], so that can make searching for things harder. Looks like they work at Valve, now? Agreed that they were a really stabilizing and healthy personality in the project, and made a lot of good early decisions, which Mercurial has continually benefited from. Eg: the commitment to backwards-compatibility [2].
[1] https://repo.mercurial-scm.org/hg-stable/rev/d4ba4d51f85f