sorry, but you are stretching the truth. the 2018 paper speculates that bacterial infections were responsible for the majority of deaths in the "spanish flu" epidemic of 1918,BUTthe authors NEVER make the claim that masks had anything to do with it. i HAVE the paper and i've read it in it's entirety. nowhere does it implicate masks. it's a small leap to conclude masks contributed, but it was NOT mentioned in the paper you cite.
furthermore, we have substantial evidence that the "spanish flu" was never influenza to begin with, but an epidemic of bacterial meningitis caused by a VACCINATION program of US soldiers who were then sent to spain to fight.
https://www.lewrockwell.com/2020/03/no_author/did-a-vaccine-experiment-on-u-s-soldiers-cause-the-spanish-flu/