I think it’s useful to distinguish knowledge of truth from gears-level understanding, these two different things can occur in any combination. Your point is that attaining specific understanding of a plan that’s good enough to make the estimate in question is a hopeless endeavor, and you list particular issues with getting such a plan fleshed out.
But it’s also possible to know truths about the world without understanding why they are true or how they came to be known (originally). The main example of this is seeking expert consensus in an area you don’t understand: by finding out what the consensus is, you get a reasonable credence in what the truth of the matter is, without necessarily understanding why it’s this way, or how specifically anyone came to know it’s this way.
This post asks for a Fermi estimate, which is another way in which a very vague model can yield truths about the world. Even if a detailed model is unattainable, such truths might be in reach.
(It’s often a lost purpose to seek truths about the world instead of seeking understanding, so it’s natural to scorn some forms of pursuit of truths. I have a lot of sympathy for this position. That doesn’t make such forms of pursuit of truths unworkable, just not relevant to improving understanding of what’s going on.)