ever used a Hoover? A biro? Sellotape?
People use flu as a generic noun - for anything from a sniffle to a chest infection. because all they have is the symptoms not the cause. Go to your Gp and they don’t know if it’s a virus or bacterial infection but pump you full of antibiotics just in case. If they don’t know from the symptoms, how does the man in the street?
Not everyone’s a virologist, although we all now think we are.
so, yes flu is a catch all term
im not going to dispute your point re the intelligence of the population, just your choice of example to demonstrate it.