I don’t have an answer either (regarding Andy’s palm muting capabilities on the low strings from this posture), but I thought it might be helpful to see how Paul does this stuff:
Also, Anton’s approach
Hand size might come into play too, since we have to remember PG has huge hands compared to small to normal sized people. I definitely don’t see the drastic shift like I’m seeing in Andy’s approach, so from the surface it seems like that’s a less desirable way to play on the low strings. Some sort of tracking or anchor/reference-point strategy that allows us to keep all the same angles makes sense to me.