Raven is Yondu's daughter? Thanatos can hear the pride in Yondu's voice, and that more than anything confirms his words. He mentally connects the two just like he had connected Yondu and Soldat. (Although he may be a bit mistaken of the nature of their relationship.)

"Death approaches no matter what the weather. In fact, weather such as this is closer to what a majority of my assignments are."

Meaning a lot of people die from the cold or cold related deaths... which are many thanks to Demeter.