While I agree there is no need to brand everything, it can serve a useful purpose, eg especially if a philosophy wants to try to spread a shift in thinking around reducing consumerism/how we live etc. This could be seen as 'tech-hippie' or other such labels, 'solarpunk' seems quite descriptive in that way. Being able to succinctly describe a philosophy that might agree with your life choices and values, it can also help reinforce those ideas to yourself in hard times as well as organize + discus with others who might share some overlap of values.
'in-groups and out-groups' and division can result from this but doesn't have to, it's not a given. Like a lot of philosophies, gate keeping via some kind of strict rules/definition I've found is usually (not always) done for other reasons by self proclaimed leaders. Being inclusive, from a community stand point, commonly is pushed aside by those that want to min/max some profitable (monetary, power, influence etc) aspect of that community causing said division. So while it might not need a name, a narrative around those ideas with a name does improve the ability to spread those values, 'solarpunk' being a brand isn't inherently bad.
We do have "very real, very big problems", ones that will need an almost unthinkable amount of cooperation to resolve. These aren't purely tech problems, these are largely social problems, and narrative/branding is immensely powerful in that sense. And all these possible solutions (technical or otherwise) are individually only going to solve small parts, slowly but surely as people work extremely hard on all these very real and hard problems, we move towards a new 'normal' without those problems, but during that long long time without those touch stones of values as reminders of _why_, it is very easy to lose focus. Narrative and branding can help solidify those ideas.
It's kind of funny in a sad way that some of the biggest companies in the world are advertising companies but when those same techniques are even slightly used and not rammed down our throats we pick up on those methods and say they are not needed. To solve those very real, very big problems, I'd say we should use everything we can to move the needle in positive direction and that 'branding', in that sense, is needed to drive the level of cooperation to address them. Maybe not the branding of 'solarpunk' but if that resonates with people, I'm all for it.