The Doylist explanation is that they hadn't thought about that back in HW. The Watsonian answer is that the planets still give off aether, and Astrology would read how said aether affects Hydaelin/Etheyris and that should give them insight into how the horde is planning to attack. This... shouldn't be how it works but it's a fantasy game and we're not given evidence that it works all the time, either. It's a translation over how real life astrology (I know, but bear with me) operates on the belief that planets have their own magnetic fields (well, this part isn't so much a belief, it's true but likely doesn't affect our lives the way Astrologists believe it to) and reading their movements can help predict... idk
Faculty of Astronomy quests spoilers
That planets give their own aetheric fields is actually confirmed in this questline, as it deals with finding a star that doesn't show up on telescopes. It's revealed that the reason it doesn't is because its light is so small or downright non-existant, that modern telescopes that use mirrors can't find it, but looking for its AETHER can locate it.
It doesn't confirm that Ishgardian Astrology can actually predict dragon movements, but it does say that they started on the right-ish track that the star (dead as it may be) is still giving off light and an aetherial field that can be seen from "our" planet.