the whole "a godless universe has low Kolmogorov complexity" argument that you're trying to make.

It might, perhaps, if I were actually trying to make that argument. But so far as I can see no one is claiming here that the universe has low komplexity. (All the atheistic argument needs is for the godless version of the universe to have lower komplexity than the godded one.)

all the universes that can support life are likely to wind up taken over by AGIs.

Even if so, you still have the locate-the-relevant-bit problem. (Even if you can just say "pick any universe", you have to find the relevant bit within that universe.) It's also not clear to me that locating universes suitable for life within something like the Tegmark multiverse is low-komplexity.

the AGI can. [...] it's likely to have a short referent to it.

An easy-to-use one, perhaps, but I see no guarantee that it'll be something easy to identify for others, which is what's relevant.

Consider humans; we're surely much simpler than a universe-spanning AGI (and also more likely to have a concept that nicely matches the human concept of "agent"; perhaps a universe-spanning AGI would instead have some elaborate range of "agent"-like concepts making fine distinctions we don't see or don't appreciate; but never mind that). Could you specify how to tell, using a human brain, whether something is an agent? (Recall that for komplexity-measuring purposes, if you do so by means of language or something then the komplexity of that language is part of the cost you pay. In fact, it's worse; you need to specify how to work out that language by looking at human brains. Similarly, if you want to say "look at the neurons located here", the thing you need to pay the komplexity-cost of is not just specifying "here" but specifying how to find "here" in a way that works for any possible human-like thing.)