Completely disagree (obviously). First off, trying to use animal ability scores as a basis for determining human(or demihuman) ability scores seems like a mistake right off the bat. Monsters, NPCs and PCs are all handled differently for a reason. Apes have an intelligence of 6 as a way of comparison to other animals to show how much smarter they are than most animals, not as a guideline for how dumb a person can be.
Second, even if you were to use it as a comparison point, who's to say that an ape can't be smarter than an impaired human? Apes in the real world are plenty smart, even to the point of learning language(sign language in this case). The limitations seem to be more of species than brain power. An ape that was smarter than most wouldn't suddenly be capable of human speech(in real life, anyway. In a fantasy setting? Maybe).
Third, using the traditional 3d6, it's possible to have an intelligence of 3 in a character who is still functional enough to be an adventurer. If 8 is Forrest Gump, then what is a 4 in terms of actually still being functional?
Finally, if you're using standard array, then 1 of your stats is going to be an 8 (barring ASI use). That means that by your reckoning, at least one stat is impaired in a way similar fashion that Forrest Gump is with intelligence. Nearly every single PC using standard array is either neurodivergent, suffering from muscular dystrophy, etc. Or else you're applying standards to intelligence that you aren't to other stats, which probably isn't great.