That number is way to high. Currently there are 35 million reported cases in the US and 624,000 deaths. So if we scale 35 million to the US population of 328 million, that is 9.4, so 624,000 * 9.4 = 5.8 million deaths, but that would be an unrealistic worst case. Reported cases is probably a significant under count which would lower the death number. Also, a significant portion of the vulnerable population has been vaccinated so we are not going to see the same death rate now as we saw earlier in the pandemic. Finally there are a lot of vaccinated people who are even less likely to die.
Letting everyone get infected will result in more deaths, but unless a far more lethal variant pops up it wouldn't be "tens of millions".