Complete anosmia with cold/flu/sinus infection is rare. Typically you can't smell anything because your nasal passages and olfactory bulb are fully occluded by excess mucus. The loss of smell is physical. If you blow your nose you can usually still smell a bit.
With COVID the loss of smell was fully neurological. It's like...a null vs. a 0. And it persisted in some cases, to the point that some Long COVID cases had to retrain their palates to fully taste food (e.g. overcoming aversion to alliums). That was the novelty.
"Long COVID-19 anosmia fatigue could result from damage to olfactory sensory neurons, leading to an augmentation in the resistance to cerebrospinal fluid outflow by the cribriform plate, and further causing congestion of the glymphatic system with subsequent toxic build-up in the brain... SARS-CoV-2 can either travel via peripheral blood vessels causing endothelial dysfunction, triggering coagulation cascade and multiple organ dysfunction, or reach the systemic circulation and take a different route to the blood–brain barrier, damaging the blood–brain barrier and leading to neuroinflammation and neuronal excitotoxicity."
With COVID the loss of smell was fully neurological. It's like...a null vs. a 0. And it persisted in some cases, to the point that some Long COVID cases had to retrain their palates to fully taste food (e.g. overcoming aversion to alliums). That was the novelty.