He has a different "net rating" in mind than you do. I wouldn't take the BR's net rating too seriously, in any case, but it's largely based on individual stats. He's referring to net rating in terms of +/- (how the team performs with him on the floor), which is arguably the most important stat (though vulnerable to small sample size, teammate effects, opponent lineup rotations). Here Gay is blowing everyone away. The Jazz have scored nearly 15 points more than the opponent/100 possessions with Gay on the floor. The next closest rotation player is Markkanen, with the Jazz outscoring opponents with him on the floor by 5 points/100 possessions. The Jazz are 22 points worse/100 possessions than the opponent when Simone is on the floor.
Now there's no guarantee that these numbers will hold in the future. In fact, I think they're unlikely to do so. But there's also no denying that the Jazz have done well with Gay on the floor this year, so far.
(By the way, those who attribute this stat to teammate effects, as I've seen some try to do, are still not accounting for the facts that Gay is both higher than all other teammates and also considerably higher. It doesn't seem logical to argue that your teammates are carrying you if you end up way higher than any of them.)