No one who is familiar with viruses considers other human coronaviruses to be "the flu". They aren't even in the same phylum! All influenza viruses are Negarnaviricota whereas all coronaviruses including SARS-CoV-2 are Pisuviricota.

There are four other endemic human coronaviruses which typically cause common cold symptoms. Those symptoms are clinically distinct from the flu in most cases.

https://doi.org/10.1016/S0262-4079(20)30862-9