Regardless of how rare we are, we'd always observe ourselves. Therefore nothing can be deduced from our own existence.
ET intelligences arising in those inaccessible places would never be able to land here on Earth.
So of course it follows that you keep saying we know nothing about aliens and must reject speculation from what we know of the laws of physics. But it's only as true as the axiom that led you to that conclusion.
It doesn't strike me as a very compelling argument. In practice, every human believes things that don't have random sample sizes that prove their existence.
I found one rebuttal here saying that applying your line of reasoning to other topics besides the existence of intelligent life leads to absurd results:
"Critics of the anthropic objection argue that Sober’s reasoning delivers highly implausible results when transferred to examples where rational inferences are less controversial. Most famous is Leslie’s firing squad (Leslie 1989: 13f.), in which a prisoner expects to be executed by a firing squad but, to his own surprise, finds himself alive after all the marksmen have fired and wonders whether they intended to miss. The firing squad scenario involves an observation selection effect because the prisoner cannot contemplate his post-execution situation unless he somehow survives the execution. His observations, in other words, are “biased” towards finding himself alive (see Juhl [2007] and Kotzen [2012] for further useful examples). Sober’s analysis, applied to the firing squad scenario, suggests that it would not be rational for the prisoner to suspect that the marksmen intended to miss (unless independent evidence suggests so) because that would mean overlooking the observation selection effect that he faces. But, as Leslie, Weisberg (2005) and Kotzen (2012) argue, this recommendation seems very implausible."
https://plato.stanford.edu/entries/fine-tuning/
Obviously we are not going to settle the matter today on which philosophers are right or wrong, but it's been fun.
The firing squad example there has other prior information that we lack when it comes to understanding our own existence. The analogy is only as good as the correspondence between the cases, and assuming that correspondence is a circular argument.
Let's break down what you wrote:
"Observer selection bias makes our own existence entirely irrelevant to the question of whether there are other intelligences out there."
This is not a "fact" but a philosophical claim about how to account for observer selection bias. Your proposed solution appears to be to shut down all discussion of the topic of what the existence of humanity tells us about the likelihood of alien life.
You also wrote:
"The firing squad example there has other prior information that we lack when it comes to understanding our own existence."
This is not a factual statement but rather a philosophical claim that the entire fields of biology and astronomy contain no relevant "other prior information" as to our understanding of our existence.