I've lost count how many species of African cichlids that I have kept over the years, and yet none of these numerous set ups were overstocked. By most MFK standards, perhaps moderately stocked, perhaps by some heavily stocked, but never "over" stocked.
My personal definition of "over stocked" is keeping fish beyond the individual set ups capacity. Hence the use of the term "over".
There is absolutely no comparison between the average mbuna tank, with fish that average under 6", with JDM style tanks with fish that average 24+", and can barely turn around, or avoid running into their tankmates when they do.
Also, most mbuna are found in the wild in large groups, so this type of setting in captivity is at least somewhat natural for these fish, unlike most of the large "monster" JDM species kept in captivity. And outside of mbuna, and a few select other cichlids such as tropheus, I don't see many experienced Rift Lake cichlid keepers suggesting to "crowd" any of the other various genus found in the Rift Lakes to help disperse aggression. Quite the opposite.
Are there idiots that over stock African cichlid tanks, of course, but that behaviour is frowned upon by the same type of fishkeepers who frown upon overstocked monster fish tanks.