That's not how I read it.
If the opponent is in hitstun for 18-20 frames, subtracted by Fox's 11 frame landing lag, he would have 7-9 frames to do something else, judging by how long will they stay in landing lag.
Fox can follow up in the 7-9 frames assuming his opponent just stands there and takes it. However if they attempt a move during hitstun, it will work. I've tested this, if they don't try to do anything, yeah, it's a full 7-9 frames, but characters are able to attack out of hitstun, like how Luigi is infamous for sex kicking out of a combo, that's a perfect demonstration.
The theory of things are indeed great, and I'm not refuting what he's saying, but with actual experience on the matter, in both normal Brawl and Brawl+, I must say that it's not black and white like that. It'll work on some characters, but not all, thus making down air with full aerial lag unreliable and inconsistent for combos, you'll be better off following up the same move, like down smash or shine. It's just as repetitive as down air and up tilt.
There is bias behind that nerf, they changed it because "it's too easy", that hardly a credible reason, considering that there are easier things that can accumulate more damage.
But that's all in the past now. The nerf has been balanced.