Is it also possible that the US have been labelling people dying of other things as COVID deaths too, as other countries have?
It would not be surprising if this number was revised later to something much lower. It is important to understand the true size of impact of COVID and this can not be done accurately when deaths are being categorised incorrectly.
This is possible. But it’s also possible that deaths are being under reported in the US and more generally worldwide.
You’d look to test this by looking at a comparison of recorded deaths versus the excess death rate, relative to the expected death based on an average.
The US official death toll is just under a million, whereas the central excess death rate is more like 1.2m.

Worldwide is shown below.
None of this is perfect, due to massive uncertainty, different reporting mechanisms, different sophistication of data from country to country. But it would be a mistake to just assume over-reporting.