Someone has Covid-19. You are exposed to the virus through them. You might catch the disease, develop symptoms, and eventually your immune system produces antibodies and you become well.
But a second scenario is also common. You are exposed to the virus. Your immune system happens to recognize the virus early, before it takes hold in your body as a disease, and produces antibodies. Those antibodies wipe out the virus, and so you never were ill with that disease. You will test positive without having been sick.
That is why there are so many persons testing positive for Covid-19 who were never ill. They never had the disease, they simply were exposed and produced antibodies early. So the claim that the number of persons with antibodies means there are many more “mild” cases than we thought is false. Those are not cases at all.
Also, when the press says that 80% of cases are mild, this is based on a Chinese study which categorized cases as “mild”, severe, or critical. Mild includes persons with pneumonia, who do not need a respirator or O2. Mild can include a set of symptoms which are very difficult for the patient to experience.
A different categorization, one that makes more sense, is mild, ordinary, severe, and critical. The ordinary category includes the persons with pneumonia.
Also, studies done in China are problematic as they may use Traditional Chinese Medicines (TCMs) as part of the standard care. TCMs include herbals which have been shown by studies to be inhibitors of the virus. So comparing a study arm with remdesivir to a control under standard care is comparing remdesivir to a set of natural inhibitors. That comparison does not tell you much. The U.S. studies on remdesivir are more revealing than the Chinese ones for that reason.