Yeah, I was a bit puzzled by that too.
Tidally locked means it's the same temperature and time of day (well our construct of time of day, anyway) all the time. But that temperature and time of day still varies depending on what part of the astronomical body you're on. There'd be a side of the Dawnlands planet that's in a frigidly cold night all of the time and a band between the hot and cold sides that is more livable (in which case, why the hell do people live on the hot side?).
There are people who think livable tidally locked planets exist, but I don't really know enough astronomy to appreciate how plausible the Dawnlands scenario is. I guess, potentially, it could be tidally locked but far from its sun, such that the sun facing side is only unpleasantly hot and not face-meltingly hot, while the non sun facing side and terminator zone are both unlivably cold. But that's a really specific scenario, especially if the planet was, at some point, not that way. (and, if that's what the alluded cataclysmic event was--
and a wizard did it--well... yeesh)
A tidally locked planet is a cool idea--
I've seen homebrewed D&D settings that have fun with it--but Red Dawn only seems to be using it as a pretext--just as it appears to be using Dark Sun's tone and aesthetic as a pretext without really appreciating or exploring the nuances.