You are mistaken. You can have pretty much any diagnosis and there is no “definitional” reason you must be symptomatic.

Pneumonia is a diagnosis (well, class of), not a symptom nor a sign, which are distinct terms of art. You can have pneumonia that shows up on a chest X-ray (a sign) without any symptoms.

A better example might be high blood pressure, which many people understand can be silent/asymptomatic for years but is easily measurable.

Point still stands, symptoms are noticeable and reportable by the patient. If covid gives you pneumonia you may still be asymptomatic.