It is also ridiculous that we now "know" so much about viruses that I have to type out that much crap in order to just say "influenza virus kicks your ass way harder than the common cold".
But once you arbitrarily declare somebody didn’t have flu if they weren’t sick for three or more days, or didn’t get a high enough fever, well then I don’t think that’s valid either in common usage or scientifically.
Since you brought up people misusing terms, I think that’s a pretty valid response.