Because there was no social distancing at that time, between 50 and 100 million people died of it. More than in the first and second world war combined.
Nonsense. Spanish flu killed so many people because it was far more deadly. Or should I say, actually deadly, as opposed to Covid-19, which many still say is similar deadliness to flu.
You seem to agree that they are totally different, except that "no people were immune". Even if that were the case (it is not), that still makes them different
This is like saying that a fast car and a slow car are exactly the same, except that the fast car has stuff inside it that makes it go faster. Total non argument.
I am not sure what game you are playing here. First line of YOUR link: "The Spanish flu [...] was an unusually deadly influenza pandemic". Not ordinary, unusual.
As for your " Only no one was immune to it", that is like saying having your head chopped off is not dangerous, except for all the stuff in your neck that gets ruined. Total nonsense
That link also talks about especially young adults dying from it, which is an extremely uncommon thing for the normal flu. It behaved in a special way.