Best understanding of why the 1918 flu hit healthy young adults so hard is that during their relatively short lives, they’d never been exposed to any influenza virus similar enough to generate antibodies that even partially neutralizd the new virus.

But most deaths were 20–40, not 0–40. Your explanation doesn’t work when you take into account the 0–19 age group, which suffered less.

I think the explanation I posted in my comment makes more sense, because it attributes the high mortality to the cytokine storm, which is most robust in that age group.