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.
solarpunk is inherently utopian. Utopias exist to inspire and to reflect on the present. It tells us that there can be a social system in which technology is good, but then if to be optimistic or not is very subjective. Many pessimistic people like utopias exactly because they highlight the ugliness of reality.