I've been watching football for a very long time. Cooper is as elite as it gets. You can argue his place among other elite WRs, but not whether or not he's elite. I think it's just motivational comments so as to not let Cooper get comfortable, pride before the fall kind of thing. This guy runs great routes and has wire to wire speed, that's elite.
I played WR throughout my entire youth. I had 4.3 speed. Never in my life have I been caught from behind. But when I look at some of the routes he runs I wish I was younger again to try them out. Some of his routes where he sells it and breaks I never even imagined when I was playing.