CFR does not have "error rate" this way. It is just defined differently then you thought it is defined. You thought it is mortality, it never was. It is the only number available at the beginning of epidemic (no matter which one).

"asymptomatic non tested population" is the most exact description of people who were not tested, had no symptoms and therefore don't count as "cases". Yes, we measure them by measuring antibodies if precise tests are available.