That's not what it says. It says that there is a range of transmission rates and proportion of serious cases that could explain the observed number of deaths in the UK and Italy. If you vary the proportion of serious cases, you could get the same number of deaths but you will be at different phases on the curve. If the proportion is really low then about 50 percent would have had is by March 15. If the proportion is really high then only a fraction have had it. They don't make a statement or prediction about how many have had it.

So in a way it's encouraging that there are plausible scenarios that are consistent with what we're observing in terms of deaths and hospitalizations that would put us closer to the end of this whole thing. However, that's just one of many possibilities. Until they can identify not just people who currently have the disease, but those who already had it, we won't know for sure.