That study included several models. As I recall, the model that predicted millions of deaths in the US based that prediction on a model that assumed no mitigation efforts whatsoever in the United States. That does not mean millions of deaths would have been the result of zero mitigation, in other words that the model prediction must be correct, but it should at least be pointed out that that was the assumption baked into that model.
Edit: just went back to look at the Imperial College study. The Imperial College group reported that if nothing was done by governments and individuals and the pandemic remained uncontrolled, then 510,000 would die in Britain and 2.2 million in the United States over the course of the outbreak.
If Britain and the United States pursued much more ambitious measures to mitigate the spread of coronavirus, to slow but not necessarily stop epidemic over the coming few months, they could reduce mortality by half, to 260,000 people in the United Kingdom and 1.1 million in the United States. Presently, this estimate does seem off, based on what has happened so far in the United States.
Finally, if the British government quickly went all-out to suppress viral spread — aiming to reverse epidemic growth and reduce the case load to a low level — then the number of dead in the country could drop to below 20,000. To do this, the researchers said, Britain would have to enforce social distancing for the entire population, isolate all cases, demand household quarantines of households where anyone is sick, and close all schools and universities — and do this not for weeks but for 12 to 18 months, until a vaccine is available.
The study did not offer estimates for the US, if the US also went with suppression as the strategy.
