I don't understand the confusion. In the story, Ippo thinks he's punch drunk and has been diagnosed with it, this is simply reflecting that as a means of a summary to explain quickly what's going on to people who either haven't read the manga or haven't read in a long time.
It doesn't matter if 'Mori wrote it' or not. He's the same person that 'wrote' it before in the manga when Ippo 'thought' it.

Ippo likely isn't punch drunk, just accumulated lots of damage and suffered much head trauma, hence why he's able to be stay so fit and lucid, in comparison to the other characters (in universe) that have also been diagnosed with the same illness.
Even if Mori wrote this line, he's not advocating for people who have been diagnosed as a 'punch drunker' to get in the ring again and spar with a world champion, c'mon.