I haven't done the AST quests yet, but consider real world parallels. You still hear astrologists in real life talking about planets going "retrograde" despite modern astronomy proving that's a load of hogwash. The planets don't shift in their paths and go backwards from time to time, it just looks like that when using a geocentric model of the solar system, rather than heliocentric. I just did a search for "retrograde" and found all sorts of doomsday sites talking about how Mercury will be going retrograde in September, 2015 and all the problems that will cause. Does the fact that the model is proven wrong cause everyone to stop using the model? No.
I'd imagine the Astrologians of FFXIV are similarly sticking to the model that they've used for a long time. Yeah, sure, there's clear evidence that one piece of it was totally wrong, but, clearly, the model has been working for them all this time, why change it? Again, I haven't done the AST questline yet, so I dunno if their star-based divinations are supposed to be accurate or just as much superstition as in the real world. But, just because they can use the power of the stars and space to cast spells, that doesn't mean everything they can do relating to the stars is true/accurate. Their spellcasting and divinations seem totally separate aspects of their work that share a common source and/or inspiration.