I mean, non-voters aren’t much more progressive really. They’re more likely to be independents (in the US at least). See:
They do skew a bit more D, but not massively so. They’re also largely non-white, less well educated and poorer. It’s a bit of a toss-up whether any of those demographics skew R or D.
I don’t really see much evidence that they’re more progressive, more centrist at best really. Although I suppose if you flatten political beliefs on a 1-dimensional axis, that does mean more progressive on average.
Do note that this differs per state, and voter turnout is also correlated with general results skewing harder in a certain direction. Complexities all around!