My guess is it came from this February study of a very small (22 patient) sample in China:

"As of Feb 27, 2020, coronavirus disease 2019 (COVID-19) has affected 47 countries and territories around the world.1 Xiaobo Yang and colleagues2 described 52 of 710 patients with confirmed COVID-19 admitted to an intensive care unit (ICU) in Wuhan, China. 29 (56%) of 52 patients were given non-invasive ventilation at ICU admission, of whom 22 (76%) required further orotracheal intubation and invasive mechanical ventilation. The ICU mortality rate among those who required non-invasive ventilation was 23 (79%) of 29 and among those who required invasive mechanical ventilation was 19 (86%) of 22.2"

I could be wrong, of course. I certainly hope the mortality rate for those on ventilators is much less. It doesn't seem to make sense to put such emphasis on ventilators if its not.