Well, I think the fact that Singer explicitly tries to tackle the problem of 'important differences' takes him out of range of the WAITW. At that point, if he fails, then his argument doesn't work. But he's not therefore doing something like 'abortion is murder'.

Edit: I just read Shulman's argument, and I think it's invalid. The fact that the drowning child and distant starving child cases differ in those respects relevant to various 'selfish' ends isn't strictly relevant to the question of their moral relationship.