His response (what I got when I read it) was targeted directly at the comment stating Bama wasn't in the playoff because Saban is slipping.
He wasn't addressing the 'Rees to Bama portion' just the 'Saban is slipping' portion.
As far as why someone as esteemed as Saban would hire Rees, I would offer two reasons
1) the opinion of Rees as an OC from the outside looking in, is much different than the opinion of us who watch every play of every game, not just the highlights.
2) with the recent QB recruiting success Rees has had (Harrman, Minchey, Carr) Saban probably wants to capitalize on that.
3) bonus reason - with the way Saban handled Kelly while at ND, and Kelly beating him in first attempt with SEC talent, Saban has likely bought into the narrative pushed by Kelly that ND was holding him back and correlates that in his mind to Rees doing better with SEC talant.