The number of deaths each year in the U.S. is highly predictable. "Excess deaths" are therefore easy to track. From late January to early October excess deaths are 299,028, per the CDC. The number of deaths attributed to COVID-19 in early October was about 200,000. If you plot on a graph excess deaths by date and COVID-19 deaths by date, the lines on the graph match up perfectly.
The conclusion is obvious. The excess deaths are mostly if not all COVID-19 deaths and we are significantly under counting COVID-19 deaths.
Excess Deaths Associated with COVID-19, by Age and Race and Ethnicity — United States, January 26–October 3, 2020 | MMWR