If he has 84% voting share, can't he just fire the board?

Firing the board is the least Matt-like option. He has absolute control and ownership over the WordPress project, I think it is more likely that he starts his own Automattic splinter company and removes Automattic from the WordPress project. I’d guess he is already filing incorporation docs for Antimatter Inc. or some other Matt themed name.

If Mullenweg attempts to fire the board that just suspended him, he would be weaponizing proxy votes against the very investors who originally granted them. While founder proxies are often structured to be irrevocable, using them to purge the board would almost certainly trigger a massive legal battle, attempts by investors to breach or revoke the proxy agreements, and severe financial instability for the company.

While he technically holds the votes required to disband the board, executing that move would ignite a corporate civil war rather than a clean transition of power.