Read your comments with interest. When you read studies, read each and every word before you.
Notice words such as: suggests, many, sometimes, and so forth.
The conclusion states,"... died from overwhelming viral pneumonia and serious complications caused by cytokine storm."
Suggests, "death from the complications that arose due to cytokine storms, in man" . . . . "how an enhanced broad immune response can sometimes worsen the outcome" of disease
The Spanish flu was brought to the USA from Soldiers returning home. I doubt the lack of Vitamin D, in that epidemic, because most folks with a little land back then needed to plant some edibles, and those on farms of even small sizes general had milk cows at least so they had milk and butter for their table, not to mention the sunshine when working outdoors for their garden, to wash clothes and hang them to dry, etc.
Vit. C may have been a little scarce in some areas, but not in many others. Let's not forget that Vit. C is found in foods other than Citrus fruits.
The Spanish flu epidemic lasted approx. 2-years before it ended, killing (if I recall) close to 500 million people worldwide. My uncle was one of them, the middle son between my Uncle other uncle and my own father.
And mustn't forget genetics has a lot to say about who gets infected, what it infects, and how bad it may be. We've this with COVID-19, but don't think a mild
case means the person is will do this well with another flu. Covid is the Novelest of the Novel viruses in the rapid manner it mutated and the way symptoms changed.
Was this the original Covid, or was it one of the hybrids with SARS, and what sort of combinations were created from this? This particular epidemic was difficult to keep up with, even for a retired Health Professional "hunkering down" and on the computer for a long shift posting pertinent finds online, or providing current information to Twitter, or Facebook; the only social media I could easily post to.
One thing to remember about Health studies. . . .the finding are all too often reversed in a few years. Consider them, look for the words that mean "maybe" and collect more data as you move on.