The anthropic principle is contingent on no additional information. For example, if sentient life exists elsewhere in the universe, your odds of being a human are vanishingly small. This would suggest sentient life does not exist elsewhere in the universe. However, given that there appears to be nothing so special about earth that it wouldn't reoccur many times among trillions and trillions of stars, we can still conclude that sentient life does likely exist elsewhere in the universe.
With an acknowledgement that on topics of this difficulty I don't expect to be right a supermajority of the time I have to disagree both on what "I am human" tells me about other beings and on what extra information tells me.
Given no additional information, noticing that I am a human increases the probability that there is sentient life elsewhere in the universe (it at least shows that sentient life is possible). It is a mistake to draw any conclusions from p(a randomly chosen sentient being is a human | there are sentient beings elsewhere in the universe). If both you and aliens exist then you and aliens exist. Knowing that you happen to be you instead of an alien isn't particularly significant.
As for extra information... well, the fact that we can't see any evidence of interstellar civilisations eating stars or otherwise messing up the place does provide weak-to-moderate evidence that intelligent life is hard to come by depending on how likely it is for intelligent life to progress that far. In that case anthropic reasoning would help explain how we could come to exist given that life was improbable. We would be an unimaginably improbable freak and all other similarly improbable freaks would be off in other Everett branches.
Assume three possible worlds, for simplicity:
A: 1 billion humans. No ETs.
B: 1 billion humans, 1 million ETs
C: 1 billion humans, 1 billion billion billion ETs.
If I am using the anthropic principle and the observation that I am human, these together provide very strong evidence that we are in either world one or world two, with a slightly stronger nudge towards world one. Where we end up after this observation depends on our priors. I agree fully that making additional inferences, such as the probability of other sentient beings increasing due to our own exi...
...at least not if you accept a certain line of anthropic argument.
Thomas Nagel famously challenged the philosophical world to come to terms with qualia in his essay "What is it Like to Be a Bat?". Bats, with sensory systems so completely different from those of humans, must have exotic bat qualia that we could never imagine. Even if we deduce all the physical principles behind echolocation, even if we could specify the movement of every atom in a bat's senses and nervous system that represents its knowledge of where an echolocated insect is, we still have no idea what it's like to feel a subjective echolocation quale.
Anthropic reasoning is the idea that you can reason conditioning on your own existence. For example, the Doomsday Argument says that you would be more likely to exist in the present day if the overall number of future humans was medium-sized instead of humongous, therefore since you exist in the present day, there must be only a medium-sized number of future humans, and the apocalypse must be nigh, for values of nigh equal to "within a few hundred years or so".
The Buddhists have a parable to motivate young seekers after enlightenment. They say - there are zillions upon zillions of insects, trillions upon trillions of lesser animals, and only a relative handful of human beings. For a reincarnating soul to be born as a human being, then, is a rare and precious gift, and an opportunity that should be seized with great enthusiasm, as it will be endless eons before it comes around again.
Whatever one thinks of reincarnation, the parable raises an interesting point. Considering the vast number of non-human animals compared to humans, the probability of being a human is vanishingly low. Therefore, chances are that if I could be an animal, I would be. This makes a strong anthropic argument that it is impossible for me to be an animal.
The phrase "for me to be an animal" may sound nonsensical, but "why am I me, rather than an animal?" is not obviously sillier than "why am I me, rather than a person from the far future?". If the doomsday argument is sufficient to prove that some catastrophe is preventing me from being one of a trillion spacefaring citizens of the colonized galaxy, this argument hints that something is preventing me from being one of a trillion bats or birds or insects.
And this could be that animals lack subjective experience. This would explain quite nicely why I'm not an animal: because you can't be an animal, any more than you can be a toaster. So Thomas Nagel can stop worrying about what it's like to be a bat, and the rest of us can eat veal and foie gras guilt-free.
But before we break out the dolphin sausages - this is a pretty weird conclusion. It suggests there's a qualitative and discontinuous difference between the nervous system of other beings and our own, not just in what capacities they have but in the way they cause experience. It should make dualists a little bit happier and materialists a little bit more confused (though it's far from knockout proof of either).
The most significant objection I can think of is that it is significant not that we are beings with experiences, but that we know we are beings with experiences and can self-identify as conscious - a distinction that applies only to humans and maybe to some species like apes and dolphins who are rare enough not to throw off the numbers. But why can't we use the reference class of conscious beings if we want to? One might as well consider it significant only that we are beings who make anthropic arguments, and imagine there will be no Doomsday but that anthropic reasoning will fall out of favor in a few decades.
But I still don't fully accept this argument, and I'd be pretty happy if someone could find a more substantial flaw in it.