The main reason to think that covid deaths are legit and not just a mistaken case of something else is that excess deaths in the US map pretty clearly with the incidence of Covid. It went up and down at the same time we got different waves. Excess deaths is a great way to look at events like pandemics after the fact.
https://www.cdc.gov/nchs/nvss/vsrr/covid19/excess_deaths.htm
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