Yes, he has a win and is a lock for the chase, so why should he try to get the best Finnish possible.
Once you have a win these races don't matter. It's only the final 10 races that matter so why bother making an effort until then.
I understand that Jr said he was worried about his head injury so he decided to hang back. He could do that because he's locked into the chase. He didn't have to try to get the best finish he could possibly get. With 'win and you're in' there is no incentive to try to get the best finish each race after you have a win.
Wish I had known he was going to run the race out riding in the back before I picked him in the race game.