All "prematurely grade this team's offseason/draft/deadline" articles are subjective to a degree. The grade doesn't bother me, though I don't agree with it.
Thus far in this offseason, the team has added Hayward, Morris, Theis, Baynes, Tatum, and a 2018/2019 first-round pick with adventurous protections. They've lost Bradley, Olynyk, Johnson, and a number of deep-bench guys, along with the opportunity to draft Fultz.
In my opinion, the trade with Philly is only a bad deal if you're so enamored with Fultz that you think having the #1 and coming away with anybody else is a failure. But if you think Tatum is in the same ballpark as a prospect, that trade is perfectly defensible or perhaps even commendable - even if the 2018/2019 pick admittedly scares the hell out of me.
With regards to the George deal, two thoughts: first is that George isn't a megastar anymore - not according to the stats. He ranked behind several SFs this year, including but not limited to Gordon Hayward. So the idea that Boston missed out on a gift-wrapped chance to acquire their alpha dog isn't exactly true. Also, there's a lot of conflicting misinformation in the media regarding that trade, to the point where I don't think we can accept any of this as 100% truth. The rumors a week ago claimed that Boston had George in the bag and wanted to resolve the Hayward situation before pulling the trigger, but wouldn't speed up their timetable when Pritchard wanted a handshake. Now Pritchard is claiming that no such deal was even in play. Believe what you want. I don't know what to think, but at the very least, I believe that treating the George situation as if Boston is the team that lost him is inappropriate.
All of this doesn't add up to 'C' to me. I'd be more in the B+/A- range, personally. But if you think passing up Fultz was a mistake, and if you think George was supposed to be in Boston and it's 100% Ainge's fault that he's not, then I could see how you would arrive at that grade.