How does the word "performative" apply to an exercise of the board's one major function? Wouldn't that be the definition of substantive?

The fact that they were subsequently fired doesn't make it into a performance. It's still a board resolution.

With 84% of voting shares by the CEO, the board was setup to fail.

I mean, they didn't fail. They were tasked with deciding if the guy was capable of doing the job. They decided that he wasn't and fired him.

Subsequent to that, they were removed from their positions. Failure is a subjective assertion that implies that they had some idea that they were going to do something different than what they actually did, but that doesn't seem supported by any of the actual facts.

Seems to me they successfully communicated that their judgment and decision was that he should be fired, and are no longer responsible for the outcome, as they've been removed from their fiduciary duty of making judgments like that.

Except, that’s far short of the full responsibilities of the board.

What exactly should they have done differently?

Resign and let minority shareholders sue, in order to have the courts decide that Matt is unfit as CEO. It wasn't the board's authority to decide the CEO was illegally abusing his powers, only courts can decide that.