It basically says that if you get a mild case of it as a kid, you'll get a less severe case of it as an adult. But we don't know how long immunity lasts. And if you're an adult that hasn't had it, then you'll more likely have a severe case. This is making the case for getting vaccinated since as an adult you can't get it as a kid under the model circumstances. It doesn't become less severe unless you had it.
"Their model predicts that if SARS-CoV-2 becomes endemic, and future generations are primarily exposed during childhood, the virus "may be no more virulent than the common cold," the authors wrote. Once endemic, the virus's infection fatality ratio, or the number of people who die compared with those who are infected, will fall below that of the
seasonal flu, the authors wrote."