(The only thing which can actually be said with any authority about those who didn't vote is that they didn't vote. How they might have voted is completely unknown. The most likely thing, statistically, is that the proportions would be the same as in the rest of the population under consideration, but that too is an assumption.)

And that's where I think we differ. I /don't/ think those proportions are the same, because the two sets of people self-select into those groups, and choosing to vote or not does NOT, IMO, have zero correlation to which way you would vote if you believed your vote mattered.