Doesn't add up.
If I recall, the Celts needed Horford's cap hold freed up to give Kemba the max.
Boston could have theoretically kept Horford and then sign and traded for Walker. Boston just had to remain below the tax line to do it, so something like Rozier (sign and traded) plus Smart, probably would have worked financially. Still might have even been able to add Kanter depending on what Horford's contract actually looked like.
So is a Walker, Brown, Hayward, Tatum, Horford with Kanter, Theis, Langford, etc. a better team. Probably, but it still isn't a contender, so I again am not sure what the point of all those moves would be. Of course I have no idea why Boston signed Walker and kept Hayward, if the goal is to build a contender around Tatum.