Others on the forum have seen the drop off without running the AC. I am not saying it is the charger, but simply not ignoring that.The heat in the cable and head are relative to the amps, not the kW going through them. TFL changed the EVGo screen a couple of times to show the volts/amps when it was at 300 kW and it showed only about 400 amps. We've seen EVGO chargers handle a full 500 amps for extended times with Rivians and other high-kWh 400 volt-based EVs. I think this is something within the EV throttling. As others noted, TFL left the EV "on" and the cabin cooling "on". If you are running the EV and have the HVAC in cooling mode and the EV thermal management prioritizes the cabin cooling, it will starve the glycol chiller and not provide full pack cooling capacity. I've seen Ioniq 5's get hit with both that situation when charging in other videos.
The Hummer will be at the high currents longer than the other EVs due to the bigger battery. All the cars are less than half the battery of the Hummer and the Rivian is 60% of it. I have no idea what level of testing has been done on these chargers, in terms of output versus time. The chillers for the cable cooling have to work hard also, and those are fairly small.