> if you change my example to an entirely different one
No, it's the same example. You said that, objectively, humans can only survive within certain temperature bands. I said that depends what you mean by "survive." Whether you can survive indefinitely at -15°C depends on whether you have a jacket. Granted, you said near absolute zero, not -15°C, but don't we also make "clothes" warm enough for outer space? Does that mean that the vacuum is as survivable as a New York winter? Kinda. We do take the availability of warm clothes for granted sometimes.
> The motivation, of course, is to say that some things fall into one category and some things fall into the other.
My point is that there are not two distinct categories here to begin with. When we call a particular belief objective, we're actually making a claim about what sort of consensus exists around the belief, whom it exists among, and how confident they are in it. That consensus depends upon a lot of things, including shared definitions and how the belief is expressed. There are degrees of objectivity, and different people will reasonably disagree about how objective a given belief is. When you say some things are simply objective and others simply aren't, you're glossing over all of that.
> facts of the matter and biological facts
When we talk about scientific "facts," we're talking about scientific consensus, which is the product of a vast social institution and an evolving body of scholarship which doesn't always agree with itself. If you'd said the Newtonian model was objective, Einstein would have proven you wrong. However, the nature of the institution of science means that physics is subjective in a very different way than how literary criticism is subjective. Physics makes reliable predictions, even when those predictions aren't 100% reliable or are based on imperfect assumptions (e.g. Newtonian physics). A binary either/or classification of "objective or subjective" leaves no room for nuances like these.
> I think for purposes of this conversation the philosophical move of taking this kind of Cartesian skeptic approach to ALL of objective reality, would throw out ALL of science, even the stuff that people would normally agree is real
I agree, and I don't think the full Cartesian approach gets us very far. However, I think if we simply lump all science together as "objective," it makes it very hard to make constructive critiques of science. Social science can make reliable prescriptions; at the same time, the replication crisis is a real thing that puts the reliability of those fields in jeopardy. At the same time, the existence of valid critiques like these doesn't mean that the consensus about (e.g.) climate change is not reliable.
> Anywho I wouldn't make a response this long if you were not giving a very thoughtful gloss on the topic
Thanks! That means a lot. Writing a good post takes a surprisingly long time.