Internal gear hubs typically (always?) can change gears when not pedaling, derailleur gears can't.

And most chain drives are derailleur systems, not IGH, so you're both right :-).

And most IGH systems are chain drive as well, so all three of you are correct :-)

if chain includes belt.

My experience with Shimano gears is that if I change when stopped or while not pedaling the change completes when I start pedaling again. The only important thing is to go easy on pedals if starting from still and don't change both the front and the rear gears.

As a category of bikes that also can’t do this is quite popular this isn’t a deal breaking issue.