Perhaps we're not thinking outside of the box enough? Maybe the term "Winterer" refers to their goals, and not the people or location. If they're meant to be the next villains and behind some kind of legitimate threat, their long term goal could be something drastic, like trying to put the whole world into some kind of hibernation/stasis for some reason. That's not too far removed from what the Endless already were, and would explain why Calyx is reporting to them.