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.'
Is that not right?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.