I think they mean that if a serious outbreak does occur, people will treat it more seriously next time (not that 20% of the population is going to die from Covid-19).
With the 1918 influenza, 1.7% of the world population died, many of them young people. This was still very traumatic to society, despite the fact that Europe had done its level best to kill young men for the previous four years in the trenches.
Right now, influenza has killed more than COVID-19, but people tend not to worry about that. Not new enough. About 300000 people a year die from influenza in a quiet year. Take your flu shots every year, people.