Don't have a ton to add that hasn't already been said, but I do think 300 watts for an hour is pretty rare for the general cycling public, or even most amateur racers.
300 watts in a vacuum, for a minute or whatever, isn't a big number. An hour is a long time. 300 for an hour is a big effort and becomes a pretty big number IMO. Really small climber dudes, hard to get the raw numbers up there. Really big dudes, raw numbers are high but sustained efforts are not generally in the toolbox for those guys.
As for Zwift specifically, total crap shoot for too many reason to list, though most have already been mentioned!
An FTP of 5w/kg for a pure amateur racer is pretty high end from my experience, a guy that can do that has an engine and while FTP isn't the end all and might not even be all that important for amateur racing at all, a guy who sits at 5 w/kg is probably pretty successful locally and likely does ok regionally in most cases.