[Question] Is Grabby Aliens built on good anthropic reasoning?

An underlying assumption of the Grabby Aliens paper by Robin Hanson et al., if I understand it, is the following:

We should expect to find ourselves as a member of a uniformly-randomly-selected civilization out of all civilizations in the history of the universe.

In other words, if there’s a master list of every civilization in the universe’s past, present, and future, our prior should be that our human civilization should be uniformly-randomly selected from that list. If you accept that assumption, then you’re obligated to perform a Bayesian update towards hypotheses-about-the-universe that predict a master-list-of-all-civilizations with the property that human civilization looks like a “typical” civilization on the list. My impression is that this assumption (and corresponding Bayesian update) is the foundation upon which the whole Grabby Aliens paper is built. (Well, that plus the assumption that there are no other Bayesian updates that need to be taken into account, which I think is dubious, but let’s leave that aside.)

If that’s right, I’m confused where this assumption comes from. When I skim discussions of anthropic reasoning (e.g. in Nick Bostrom’s book, and on lesswrong, and on Joe Carlsmith’s blog, etc.), I see lots of discussion about “SIA” and “SSA” and “UDASSA” and so on. But the Grabby Aliens assumption above seems to be none of those things—in fact, it seems to require strongly rejecting all of them! (E.g., note how the Grabby Aliens assumption does not weight civilizations by their population.)

I feel like I’m missing something. I feel like there are a bunch of people who have spent a bunch of time thinking about anthropics (I’m not one of them), and who endorse some “standard” anthropic reasoning framework like SIA or SSA or UDASSA or whatever. Do all those people think that the Grabby Aliens paper is a bunch of baloney? If so, have they written about that anywhere? Or am I wrong that they’re contradictory? (Or conversely, has anyone tried to spell out in detail why the Grabby Aliens anthropic assumption above is a good assumption?)