I'm not CFB, but live I thought it was clearly grounding. No receiver in the area, in the pocket, under duress. In fact, as soon as I saw Brady run toward the ump to assert there was PI, I thought immediately, "I know what he's doing, he's trying to lobby to get out of an IG call." CFB or others can tell us, but I don't think "I thought the receiver would run a different route" doesn't seem to be a defense under the rule.
Brady throws the ball away more from inside the pocket than any other current QB I have seen. Most QBs get out of the tackle box when they throw it away or they throw it 15 feet high out of the back of the end zone, which never seems to get called. I've actually thought there were some additional times he could have gotten called this year, like when he spikes the ball 10 feet short of a running back who has his back turned and the refs give him the benefit of the doubt.
The lore seems to be that nobody else gets called like Brady gets called, but I just never see anyone else with so many throw always as Brady, except maybe Rodgers, but he's more mobile and can usually take a couple of steps left or right before he does it. Brady is a master at throwaways -- best I've ever seen at feeling pressure and avoiding negative plays. He has almost perfected it. He's going to take some calls. Given that it's no worse, really, than a sack, and certainly better than an interception, I can live with it.
I could also live with them getting rid of the rule. I think it's a dumb rule, especially in the era of QB safety. It's a sack-encouraging rule. Why are we encouraging sacks? I could see if the rule was that it's unsporting to throw the ball away, so you're not allowed to do it, but that's not the rule. The rule is that you can do it now, just so long as you take a couple steps left or right. Why should the intentional grounding rule favor mobile quarterbacks? Make it a legal play or don't.