LW - Am I confused about the "malign universal prior" argument? by nostalgebraist

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Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Am I confused about the "malign universal prior" argument?, published by nostalgebraist on August 28, 2024 on LessWrong.
In a 2016 blog post, Paul Christiano argued that the universal prior (hereafter "UP") may be "malign." His argument has received a lot of follow-up discussion, e.g. in
Mark Xu's The Solomonoff Prior is Malign
Charlie Steiner's The Solomonoff prior is malign. It's not a big deal.
among other posts.
This argument never made sense to me. The reason it doesn't make sense to me is pretty simple, but I haven't seen it mentioned explicitly in any of the ensuing discussion.
This leaves me feeling like either I am misunderstanding the argument in a pretty fundamental way, or that there is a problem with the argument that has gotten little attention from the argument's critics (in which case I don't understand why).
I would like to know which of these is the case, and correct my misunderstanding if it exists, hence this post.
(Note: In 2018 I wrote a comment on the original post where I tried to state one of my objections to my argument, though I don't feel I expressed myself especially well there.)
UP-using "universes" and simulatable "universes"
The argument for malignity involves reasoning beings, instantiated in Turing machines (TMs), which try to influence the content of the UP in order to affect other beings who are making decisions using the UP.
Famously, the UP is uncomputable.
This means the TMs (and reasoning beings inside the TMs) will not be able to use[1] the UP themselves, or simulate anyone else using the UP. At least not if we take "using the UP" in a strict and literal sense.
Thus, I am unsure how to interpret claims (which are common in presentations of the argument) about TMs "searching for universes where the UP is used" or the like.
For example, from Mark Xu's "The Solomonoff Prior is Malign":
In particular, this suggests a good strategy for consequentialists: find a universe that is using a version of the Solomonoff prior that has a very short description of the particular universe the consequentialists find themselves in.
Or, from Christiano's original post:
So the first step is getting our foot in the door - having control over the parts of the universal prior that are being used to make important decisions.
This means looking across the universes we care about, and searching for spots within those universe where someone is using the universal prior to make important decisions. In particular, we want to find places where someone is using a version of the universal prior that puts a lot of mass on the particular universe that we are living in, because those are the places where we have the most leverage.
Then the strategy is to implement a distribution over all of those spots, weighted by something like their importance to us (times the fraction of mass they give to the particular universe we are in and the particular channel we are using). That is, we pick one of those spots at random and then read off our subjective distribution over the sequence of bits that will be observed at that spot (which is likely to involve running actual simulations).
What exactly are these "universes" that are being searched over? We have two options:
1. They are not computable universes. They permit hypercomputation that can leverage the "actual" UP, in its full uncomputable glory, without approximation.
2. They are computible universes. Thus the UP cannot be used in them. But maybe there is some computible thing that resembles or approximates the UP, and gets used in these universes.
Option 1 seems hard to square with the talk about TMs "searching for" universes or "simulating" universes. A TM can't do such things to the universes of option 1.
Hence, the argument is presumably about option 2.
That is, although we are trying to reason about the content of...