Yeah, I can see where it may be better than expected... the biggest issue is they still don't know what the true "percentage of fatality" is, they don't know what sorts of things to look for to predict when it drives the immune system into overdrive to cause the fatality when the pneumonia doesn't do one in, and even if having the antibodies confers any sort of even short-term immunity (technically it's still untested, there's admissions that some of the tests are weighted towards false-positives for safety's sake, it hasn't been long enough with any verified recoveries, and with lock-downs going on, who knows when we'll actually begin to find out), 21% isn't enough for herd immunity. There's still a chance that it could overwhelm an area's hospital system (in whatever wave).
"You know how parents tell you everything's going to fine, but you know they're lying to make you feel better? Everything's going to be fine." - The Doctor