As I understood it, the dashed line is just the 35°C wet bulb temperature line.
I think it's the "old assumed border of survivability" and don't know if it is based solely on mathematics or on other experiments as well.
I also don't know on how many individuals the new line is based and what age group the older people one is.
As always I will keep reading about every year's COP. However, by now my expectation is, that there won't be much, if anything at all, that I need to know about the COP.