From the article (as well as other places I've read similar things).

"Why did so many young adults die? As it happens, young adults have the strongest immune systems, which attacked the virus with every weapon possible—including chemicals called cytokines and other microbe-fighting toxins—and the battlefield was the lung. These “cytokine storms” further damaged the patient’s own tissue."

The more robust immune response was a large cause of the increased mortality.

Iirc from my time as a biologist, we don't have any direct evidence that a cytokine storm happened and there seem to be other possible explanations for that distribution.

The belief is that it was disease related. Younger, healthier people have more active immune systems and the Spanish flu was causing it to kick into overdrive (similar to a cytokine storm).

I think the war definitely helped spread it as living conditions were rough and there was plenty of migration.