Wood frogs can survive freezing solid. Their liver produces glucose to flood all cells, prevent cell freezing, and protect against dehydration. Ice forms around cells and organs but not inside them, preventing lethal damage.
And if the book was about how such a "virus" might work, the author could have brought that up. But since the book is actually exploring how different cultures or people groups might react to the same event, going down that road would be a trap, and only invite more intense scrutiny from pedants. Explicitly "lampshading", as a sibling commenter put it, is the better way to achieve the author's goals.