If you do a serology study in California and it estimates that the COVID-19 infection fatality rate is 0.10%, you need to come up with an explanation why 0.12% of all of New York City has already died.
Yes but excess all cause mortality will include things like "had a heart attack, didn't seek treatment due to fear of COVID-19 at the hospital, died" which aren't relevant to the epi characteristics of the virus. NYC seems to be doing a decent job counting COVID-19-like deaths.
