The 1918 flu was the origin of one strain of seasonal flu, but others are older. The word "influenza" itself comes from an Italian phrase "influenza di freddo" which bascially means "influence of cold" which goes back to the 1300's. As the name indicates, it was associated with cold weather even then. It may have been described even earlier by Hippocrates, but it's hard to say if that was really the flu as we know it. After all, many respiratory diseases are associated with cold weather and cause similar diseases. There were some similar (but less intense) flu epidemics documented back to the late 1800's.

Article on the topic: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(19)30148-5/fulltext

Still, influenza is a pretty fast moving disease. We can get some idea how old individual strains are using molecular clocks. I'm used to reading about these dating animal lineages, when the timespans are in the hundreds of thousands or millions of years. With flu, we are talking about timespans of decades. All three currently circulating influenza A strains first appear in the 20th century, and the deepest branches showing up on the tree in the linked paper only go back to the 1800's! That doesn't mean flu wasn't around before then, just that each gene they looked at came from a common ancestor at about that time.

https://www.pnas.org/content/106/28/11709

Divergence times between strains are all over the map. Influenza A and B have been estimated to have diverged a few thousand years ago, a few hundred years ago, or even about a hundred years ago, but I couldn't find a really recent paper. Here's a couple the first promoting a long timespan and the second a short one.

https://academic.oup.com/mbe/article/19/4/501/995507

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2443812/

You're misreading that section as saying that seasonal flu didn't exist before 1918. What it's actually saying is that the 1918 flu became the seasonal flu, by replacing the seasonal influenza virus(es) that were circulating at the time. All the seasonal influenzas we are now (2021) infected with are direct descendants of the 1918 virus. See How did the Spanish flu pandemic end? for more detail.

Before 1918, influenza was well known (although probably often confused with other diseases - the cause of influenza wasn't identified until 1934), and it was well known to be mainly a winter disease. This was muddled by pandemic influenzas, which often spread outside the normal season, but in general influenza was known to be a winter disease. For example, if you look at Table 9 ("DEATHS (EXCLUSIVE OF STILLBIRTHS), FROM CERTAIN CAUSES, BY MONTHS, FOR THE REGISTRATION AREA, REGISTRATION STATES, AND CITIES OF 250,000 POPULATION OR OVER IN 1910") from the US Mortality Statistics from 1910 you can see that in Philadelphia in 1910, 150 of the 159 influenza deaths occurred between November and April, with none in July or August.

This was so well known that some textbooks warned that it was far too easy to simply ascribe any winter death or disease to influenza. Little pamphlets about family health explained that influenza season was closed-window season, and he prevention for influenza was to open windows.

Flu was a thing before 1918, but in 1918 there was a new strain of H1N1 which most people had no immunity to (was zoonotic and had just crossed from birds to humans). If I remember correctly some old people did as they was a similar virus like 30 years earlier. Hence part of the reason the death rate was lower in the elderly