Ok, a few things:
a) correlation does not equal causation - BMac has had a golden run over the last 18 months in test cricket at precisely the same time as he's taken even more of a T20 style approach to his batting than he had previously.
b) Not all players are the same: just because Clarke may have benefitted from quitting T20's, doesn't mean that the same will hold true for everyone. Plenty of top-level batsmen from a number of countries (Sangakkara, Jayawardene, Kohli, Root etc.) have played and continue to play both tests and T20's to an extremely high standard. Furthermore, Williamson plays a lot less cricket in all formats than Clarke did, so the difficulties of switching back and forth between the formats in short timeframes is likely to be less severe on him.
c) We've yet to see any negative impact on Williamson's batting in any formats. He played a couple of pure, well-paced, classic-KW ODI-innings in the last couple of weeks (right down to being dismissed in the 90's), and he recently scored a superb hundred at Lords in a test-match against a high quality bowling unit. Until there's some evidence that T20 cricket is actually affecting his batting negatively (and I have to say that given his standards of professionalism, KW is likely to be a far better judge of that than you or me) I see absolutely no reason why he should restrict himself from playing T20's.
d) Just because he played a cross-batted slog at one stage doesn't mean that that's a sign that his skills are deteriorating. Ross Taylor was carving up in test cricket at the same time as he was playing his cow corner hockey-slog every other ball in the IPL.
So yeah, this strikes me as an overreaction to the continued expansion of KW's skills as a batsman. Furthermore - and I concede that this is tangential to the debate at hand - expecting him to give up T20 cricket like Clarke did is a hugely unrealistic expectation to have. Clarke could afford to quit T20's because he gets paid $1+ million by Cricket Australia. KW gets maybe $250k from NZ cricket, so it's only reasonable to expect that he's going to look to take advantage of the IPL and other T20 money-making opportunities where they arise. Given that he'll already be playing a lot of T20 cricket anyway, I don't really see the value in him cutting out a half dozen or so T20I's from his calendar (besides maybe the obvious benefit of getting a rest every now and then).