I don't know if most people experience anosmia like you describe while having a common cold or the flu, though (I think they don't). From what I've read this symptom is a pretty reliable indicator of covid19, so much so that at this time it pretty much rules out any other diagnosis and it is a strong basis for medical decision making. (Of course, there will always be outliers for any symptom. Decisions still need to be made at the population scale, though).