The Mylar balloon in the video is going faster than the jet. That seems incredible, no?

Is this the case where smarter people actually figured out the velocity was just a few meters/second when properly taking into account everything? I have trouble keeping up.

It's "incredible" in the sense of "don't believe it without more reliable evidence"

?

where is there a direct measurement of the speed?

So pilots are able to identify Bart Simpson balloons when they take the time to investigate something. That makes David Fravor and other pilots stories even more compelling when they were looking right at the object for minutes and what they saw wasn't explainable.

Mylar balloons drift about. They quickly blow away in hurricane force winds

Look at the videos linked. These were perfectly stationary in hurricane force winds, and shot away to one side at the speed of sound.