20% is definitely a lot. But if you look at the estimates of the death rate in Cambodia after the Khmer Rouge, they range from a bit over one million to almost three and a half in a population of roughly eight. So it does seem doable.
Also, Khmer Rouge had like 4 years to kill the people, and big waves of covid-19 last approximately one or two months. For example famine and unrelated illness that can't be treated because the hospitals are not working usually take more time to kill people.