This trade was more about freeing up cap next year, then it was getting a player that was worth moving Kemba in return. Maybe you could have gotten decent role player, but would have taken on probably a bad contract.
I would had thought Horford would be moved around draft night, like to the GSW, but listening to his sister who seems like she's setting up in Boston now, maybe he's here for a year, and then he's on the bench as a coach the next after they buy him out instead.
This deal may have not been there later, there will be some teams that are trying to get some wiggle room next summer with it being a decent FA, and the one after that too.