So I'm gonna sound jaded but this isn't news to me. It was never in an "official" capacity (student at best) but this "result" turns out in interviews, discussions, in social sciences (cognitive psychology first of all, but certainly anthropology as well, philosophy too, it's a cross-domain topic). You just have to listen carefully and someday you hit this weird fact.

It's too long to explain here, but there's another realization that somehow helps describing such discrepancies between human beings. The question is that of 'normalcy', versus the 'abnormal', the notion of 'deviance', which leads to 'pathological', 'syndroms' and other supposedly 'undesirable' mutations.

Well, the history of evolution itself is that, among a population of "normal" beings, the next stage for any species always begins with 1 mutation, 1 "freak" who has a new thing, and will eventually spread to the whole species if that trait is worth it for survival and better adaptation.

There are enough accounts of historical psychology research to affirm that biologically, cognitively, psychologically speaking, there's virtually no difference between human beings 4,000 years ago and today; a baby from then raised now or vice versa would grow up just normal in context — we know this because every witness from the past said so, there's a recurring chain of normalcy if you will, and reading Ancient texts yields fairly "normal" psychological profiles. (Also think that 1,000 years is but 30 generations or so.)

And yet there are tests measuring dramatically changing cognitive abilities over just a century — take attention span in the last 40 years, it's appaling.

This means, IMHO, that minute change keeps occurring much more than we might suspect, continuously, but over long enough periods that human history is dwarfed by meaningful special (species) evolution. It also means that right now, a minority among us have evolved to what is the next stage for all of us, but it will only be obvious in hindsight.

The 18 over 91 quoted by OP has having no spoken dialectic (rather through other forms of encoding than language/speech) may be a dying breed, an artefact from the past (the animal in us who hasn't fully integrated speech maybe); they might as well be the future, how human brains would evolve as they adapt ("beyond-speech humans" in that case). Maybe the real path is people who have the ability for both, a switch.

A fair point was made in a previous to-level comment about the "speed limit" of speech, in terms of bandwidth or information density, and this is echoing how e.g. the math genius in us tends to think, when notation becomes object beyond than abstraction in and of itself and there are no words anymore but concepts, often impossible to visualize anyway (e.g. it's not a "function" or "f" anymore, in context it's become a "..." — no word but a feeling of what it is and what it does and how we could manipulate this specific thing).

I guess the take away is there are many discrepancies that we would never suspect had we never talked to others, and a lot of what we take for "granted" about human nature is typically a strong hint for something that's very subjective. At least in perception, if not in value. And that while we are entitled to our own hierarchy of values in the present, we simply cannot possibly judge the endgame, we can't tell today who's embodying the shape of things to come — just that some of us are, some of us must, if we are indeed evolving.