I think the concern is not the fact he has slipped on D (understandable for the reasons you mention), but the degree to which he has slipped on D.Generally speaking, the larger role one takes on offensively you are going to lose some defensively from conserving energy. I can’t think of a big scorer who hasn’t picked spots throughout the course of the game. These breaks will increase with age too so to expect 30 ppg on one end and lock down on every regular season possession on the other isn’t a realistic expectation.
It doesn’t help that Tatum doesn’t yet have his defensive role player to best allow those breaks to be hidden.
Advanced stats are not gospel, but we are beyond small sample size now with Tatum and his bad D. The slippage has been extremely dramatic, far more than you would expect for a young healthy player before his prime.
I think he has the tools to get closer to where he was, but Tatum's D is still a big concern until he shows otherwise.