The difference I was getting at is that when constructing a code you're taking experiences you've already had and then assigning them weight, whereas the universal prior, being a prior, assigns weight to strings without any reference to your experiences. So when people say "the universal prior says that Maxwell's equations are simple and Zeus is complex", what they actually mean is that in their experience mathematical descriptions of natural phenomena have proved more fruitful than descriptions that involve agents; the universal prior has nothing to do with this, and invoking it is dangerous as it encourages double-counting of evidence: "this explanation is more probable because it is simpler, and I know it's simpler because it's more probable". When in fact the relationship between simplicity and probability is tautologous, not mutually reinforcing.

This error really bothers me, because aside from its incorrectness it's using technical mathematics in a surface way as a blunt weapon verbose argument that makes people unfamiliar with the math feel like they're not getting something that they shouldn't in fact get nor need to understand.

(I've swept the problem of "which prefix do I use?" under the rug because there are no AIT tools to deal with that and so if you want to talk about the problem of prefixes, you should do so separately from invoking AIT for some everyday hermeneutic problem. Generally if you're invoking AIT for some object-level hermeneutic problem you're Doing It Wrong, as has been explained most clearly by cousin_it.)