I understand their arguments just fine. I just don't think they make any sense.

Ought implies can. We cannot predict the far future of humanity. We cannot colonize other planets in the foreseeable future. We cannot plan how to handle future technology that we aren't yet sure is even possible.

The things we actually can predict and control, like global warming and natural disasters and pandemics, are handled with regular old public policy. Longtermism, almost by definition, refers to things we can neither predict nor control.