I see the symmetry between P1 and a universally compelling moral argument in this: they both make a claim about the application of an argument quantifying over all minds in mind-space.
The claim EY is refuting is 'For all minds m, m: (moral argument X is compelling)m.'
P1 makes the claim 'For all minds m, m:(an argument of the form 'for all minds m:X(m) is unlikely to be true)m.'
UCMA is making a claim about all minds, P1 is making a claim about some undefined subset of all minds.
They both talk about "all minds," but only one of them makes a claim -about- all minds.
A parallel pair of arguments might be: All squares are rectangles The claim that all squares are rectangles is unlikely to be true of all squares.
The first claim is stronger than the second, and requires more proof. The fact that we can in fact prove it is irrelevant, and part of why I chose this example; consider the inverse propositions that all rectangles are squares, and that that claim is unlikely to be true, to see why this is important.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.