Right, but it isn't a point about 'quality' of competition only, it is also about matchups. If you play 35 minutes you're out there for many different lineups for your team and the competition. If you play 20 a game (like Amir did last year) you see many fewer, and a decent coach is going to spot your against situations that fit your skills better. That will tend over a season to give you better plus minus numbers, but those numbers wouldn't scale up in more minutes---in fact we'd expect them to go down a lot. That's an issue in valuing the player
I do think Amir has value and is better than the eye test...and also that folks who put too much weight on plus minus related stats are overvaluing him a bunch too
Not to mention the conditioning. You can probably play with more intensity when you are only out there for 20 minutes a game. You don't have to worry about foul trouble either.