Earlier quoted context omitted.
The number of humans that can live on this planet simultaneously is a function of technology. If we all lived like hunters and gatherers, the population limit would be a mere few tens of millions. Conversely, if we imagine arbitrary technology, the maximum population that can live simultaneously and sustainably on this world is somewhere in the trillions. Technology such as the stuff you’re trying to use as evidence…
It's also a function of the finite resources available to us. You also need to factor in that we don't have arbitrary technology and never will. There will always be limits to our technology. I think the gist of the GP argument is something like: Our use of technology is using increasing amounts of our planets resources (e.g. land), and is often questionably ethical (exploiting the other creatures we share our planet…
But we don’t have trillions of people yet, and the difficulty of supporting the current population sustainably is more one of organisation than technological inability — much closer to irrigating and enclosing in greenhouses Nevada, Utah, and Arizona [0] than it is to colonising Mars. (I’m told greenhouses are way more productive than open air farms, just not cost effective in most cases; the example given by proponents is the Netherlands).
> is an expanded human population worth that cost?
To whom? I’m vegetarian and pushing myself in the direction of veganism, partly because of ethics. Ethics are however are relative, and many humans only care about other humans. Heck, given that the nature of the rich inner world often called “self-awareness” or “consciousness” is not yet sufficiently well defined to even be testable, such people may even be correct. I’m not even sure at what threshold to stop caring myself — lobster brains aren’t very complex, and bivalves (despite being animals) don’t even have brains: https://en.wikipedia.org/wiki/Bivalvia#Nervous_system
[0] for illustration purposes only; this is almost certainly a suboptimal set of locations.