Am I missing something basic here?
No you’re not missing anything that I can see. When OP says:
Does longtermism mean ignoring current suffering until the heat death of the universe?
I think they’re really asking:
Does longtermism mean ignoring current suffering until near the heat death of the universe?
Certainly the closer an impartial altruist is to heat death the less forward-looking the altruist needs to be.
No you’re not missing anything that I can see. When OP says:
I think they’re really asking:
Certainly the closer an impartial altruist is to heat death the less forward-looking the altruist needs to be.