What if engagements around that user naturally declined, perhaps due to that user going off the deep end. Wouldn’t this just serve to bias the algorithm toward propping up the exposure of that user? Do they even care about the control so long as that user’s engagement is up and to the right?

Come on think this through. It’s trivial to tell the difference between a gradual and natural decline and a drastic decline immediately after rolling out a change. Especially when the change is rolled out region by region and only exists in regions running the update. You have to be able to measure the effect of changes and the most popular accounts are the obvious low hanging fruit for doing that.

You do an A/B test so even for the same tweet or same time period, you’re just comparing the new, “treatment” group against the old “control group.”

He's very likely the user on the platform with the most engagement, and probably by a long distance.

From that viewpoint, it does make some sense to use his account as measurement point.