Wow, you just showed me something profound. I wonder why news are not covering this. Anyways, I find it puzzling as well.I always assume it spread like any other respiratory pathogen.

Other research papers that cited the Rosenau (1919) paper either said it's a mystery or that the volunteers were immune:

"Some of the earliest published human challenge

experiments with respiratory viruses took place during the 1918–19 influenza pandemic when attempts were made to show transmission of infection from symptomatic patients with presumed influenza to healthy volunteers.15 These experiments were unsuccessful, probably because the volunteers were immune." -from Killingley et al. (2011).

https://www.sciencedirect.com/science/article/abs/pii/S1473309911701426

It's not open-access so feel free to email me for the full-text. Also, I think I might want to write about this. Hope you don't mind I take this idea :)