For what it's worth, the US CDC has said this about (H1N1)pdm09. I'm not sure if H1N1 is a coronavirus though and I don't know if there's any published research supporting this. But the author's statement seems reasonable enough given it was qualified with "probably".
> From April 12, 2009 to April 10, 2010, CDC estimated there were 60.8 million cases (range: 43.3-89.3 million), 274,304 hospitalizations (range: 195,086-402,719), and 12,469 deaths (range: 8868-18,306) in the United States due to the (H1N1)pdm09 virus.
That is untrue. There are coronavirus outbreaks all the time. What makes MERS and SARS special is that they are the only known human coronviruses with high mortality.
> From April 12, 2009 to April 10, 2010, CDC estimated there were 60.8 million cases (range: 43.3-89.3 million), 274,304 hospitalizations (range: 195,086-402,719), and 12,469 deaths (range: 8868-18,306) in the United States due to the (H1N1)pdm09 virus.
https://www.cdc.gov/flu/pandemic-resources/2009-h1n1-pandemi...