It had to be true. The npc's say it works. And Middy directly mentions it as well while the star was shining in the background.

Either it's just a dragon cultural thing they made up living on earth, deciding the best course of action depending on the pulse of their star for warrior goodluck. Or they have some kind of boost from their star similarly to sharlayan Astrologian using aether from space. So they always exploit that advantage for attacks.

Keep in mind, that light/aether may take millions of years to reach the planet but it reaches the exact same time a dragon and astrologian observes them. Astrologian at least has tools to predict and map the likelyhood the star is at it's "prime" beforehand