Solarpunk usually describe societies where the negative aspects have just disapeared, and been replaced by stuff that the genre proponents enjoy. From what I read, the claim is that the "punk" aspect is the rebellion against negativity. But that's not something we can see in their production.
Anticapitalism isn't punk in a world where the anticapitalists win.
The era of collapse fiction like cyberpunk has only really given rise to one political movement: accelerationism. Whose defining trait is "if the world's going to collapse, we should probably hurry it along so we can see if there's anything to salvage afterwards".
Punk was never about collapse, you're confusing that with post-apo.
Also I'm unconvinced about the political potential of solar utopias: they're usually centered on art, and completely avoid the how. So many of them draw nice-looking landscapes that make no sense: glass and steel have to come from somewhere, urban centers can't exist without a periphery, etc. All the issues are carefully avoided and labeled as "negative thinking, which [authors] rebel against".