I've no idea how legit, or popular, this notion of longtermism is.
But it sounds like a great excuse to ignore almost all of the world's current or "in our lifetimes" problems, focus on a few very theoretical problems - of serious interest to only a few SciFi fans and longtermists - and proclaim it a virtue to do that. (While probably doing nothing whatever about any problem, beyond talking big and puffing themselves up.)