While it's true that the mortality rate for flu is not based on serology testing, the point is that scientists estimate roughly 1 in a thousand flu victims dies from the virus, and scientists also estimate roughly the same proportion of CIVID-19 victims die from that virus. I trust the science.
Would be interested to see a source on this that actually puts it around 0.1% of people that contract the flu virus dying.
It's horribly worded and confusing, but the CDCs method that puts at about 0.1% does not try to include asymptomatic and mildly symptomatic people that an anitbody test would capture (IFR).
Put it this way, if a true IFR from the flu was 0.1%, and the mid-point estimate for deaths from flu by CDC of 40,000 in the US is accurate, that means only about 12% of the population contracted the flu virus regardless of symptoms being present or severe.
That could be true, but if under business as usual only 12% of people are infected by seasonal flu, all data supports the fact that under business as usual a much higher percentage of people would become infected with the novel coronovirus.
So no matter what, the conclusion is that under the same circumstances (business as usual) COVID-19 is likely at least several times more deadly than the seasonal flu. The limitations in the data simply mean we don't know how much of the additional danger is due to the effects of the disease itself vs. due to the increased transmittability.