The core difference between longtermism and solarpunk is that longtermism stems from an utilitarian frame, while solarpunk rejects it. Radical utilitarianism like longtermist fashos and oligarchs gives them a way out to commit the worst crimes against humanity because of a supposed good that will materialize in a distant future. It’s a moral free pass, exploiting the life of future humans (who cannot protest) to justify the oppression and exploitation of current humans (who are indeed protesting these assholes).

Solarpunk and longtermism are in no way on the same spectrum.