It doesn't need to travel faster than light. The Fed evacuation fleet has years to complete the evacuation. Also, about 'without the warning,'we now a have a reason to suspect that Betelgeuse will go supernova soon. That 'soon' may mean tomorrow or it may mean in hundred thousand years. So even with better tech it might not be exact science. The Romulan scientists may have indeed known that the star will explode within next couple of millennia, and the empire may still have not taken necessary precautions. You don't need to look far to see how that sort of insanity could happen. Supernova talk gets branded as leftist fear mongering, and the Praetor promises to 'make the Romulus great again' instead of spending immeasurable resources and upend peoples lives to prepare for a calamity that probably doesn't happen during his lifetime. And people buy that, as the truth would be too uncomfortable and accepting it would mean making serious changes to their lifestyles.