There will always be small numbers of cases of stupid coroners, clerical mistakes, etc. Finding one is not proof that there are hundreds of thousands.

We can rule out the "they're reporting everything as COVID and that's making the numbers high" hypothesis by looking at overall excess deaths - how many more deaths is the nation experiencing than is typical. A misdiagnosis won't affect excess deaths, as it counts all deaths, regardless of cause. The CDC provides a page for this:

https://www.cdc.gov/nchs/nvss/vsrr/covid19/excess_deaths.htm

As is readily visible in the chart, we've been substantially over the norm since late March.

(Switch the chart to New York City for an even more obvious example of how a spike in excess deaths shows up. Screenshots: https://imgur.com/a/zSkNbxT)