That doesn't mean the flu doesn't exist.
It means that influenza weakened their lungs so much that they were susceptible to pneumonia which eventually killed them.
The 1918 Spanish flu was especially good at weakening people to the point where other infections and illnesses could kill them.
Sick with flu, die with pneumonia. Same thing with Covid. Sick with diabetes, heart condition, etc. die with Covid. At any rate without the flu in 1918 or the Covid now, far fewer deaths would have/will occur. People should get the pneumonia vaccines that are available today. I took one about 10 years ago and an improved one a few years ago. No bad symptoms either time. Tender shoulder a day or two is all. The pneumonia danger is why Dr. Zelenko’s 3 part Covid protocol included lung protecting Azithromycin (Doxycycline may be better for some).