> Whereas many definitions I've seen for gaming disorders set a threshold of 20 hours a week or 2 hours a day to define gaming disorder.
Neither the new ICD-11 “gaming disorder” nor the DSM-5 “internet gaming disorder” (the latter identified as a condition for further study, not an official diagnosis, but with diagnostics criteria) have any number of hours criteria.
> Seems pretty likely that the concern over gaming is primarily due to it being a new medium.
Gambling isn't a new medium, but both DSM and ICD recognize problem gambling (“gambling disorder” and “pathological gambling”, respectively.)
> I fail to see how staring at a screen passively for 4-5 hours can be considered normal, while staring at a screen while actively facing challenges and solving problems for half that time is a disorder.
The latter wouldn't, in and of itself, be a disorder under either the DSM or ICD criteria related to internet gaming disorder (DSM) or gaming disorder (ICD). So, sure, you're right, but only because the hypothetical you pose is completely irrelevant.