Nor have I, it sounds fairly arbitrary to me. I don't think a global society that's structured the proper way need have a maximum population limit. If energy becomes cheaper and cheaper, like with breakthroughs in solar, wind, nuclear, storage, fusion, then it seems to me we could easily support 10 billion people's food and resource needs, given a circular economy which is highly efficient. There's also asteroid mining, carbon capture, vertical farming, high density living like in Asia...there are a lot of efficiencies to be gained in how our population lives.
It's interesting to think about. For instance, how much space do we need per capita? If we were to pack everyone together giving each a 2ft by 2ft space, the current population would take up the space of a large county or two. That's rather fascinating to think about because it isn't a large amount of space. Obviously it isn't sustainable, but our physical size is small. But then you start factoring in food, shelter, water, mental well-being, etc... It would just be interesting to find out how much space we really need per capita thinking along those lines.
It's referenced in the Countdown book I mention, which includes citations. The author excludes the book from the Internet Archive OpenLibrary, so I can't directly cite the page. I'll have to reply back later when I can grab the book and find the page and citation it refers back to.
Where does this come? Is it referenced in the books you mention? I just have never heard this before.