Why? Because, let's face it, on a scale of 1-10, 4 and 3 at least, are not totally mentally handicapped. Hell on average 4-5 at the most, is the general intelligence in most people irl, prove me wrong look at history .
I'm unsure as to what you are suggesting here, but I'll humor you.
On a scale of 1-10, 5 is average, making your statement senseless. And even if the "average intelligence" was 4, that would be immediately be remedied -- as in in the moment the average intelligence score dropped, it would trigger an enforcement of the aforementioned logic -- because 4 (below average) would therefore become 5 (average). You even go on to say right after the above quote, 5 is "general"... which is... synonymous of "average".
I think all what he's saying is that chars just below average (ie.. 4 only imho) shouldn't be "retarded" and talk stoopid.