It could also be referring to the entire star system that Hydaelyn resides in. Referring to the planet, moon, star and other planets.
Isn't there some lore that states the first dragons came from space? And Isn't it a dragon that says the specific phrase in question? In which case it's possible that the dragons still recognise Hydaelyn as just a small part of the star system.

Off-note: you wouldn't generally refer to the planet you're currently on as "this planet" but rather "Earth" (that's not technically the name of OUR planet, just a term used to refer to it, if you landed on another earth-like planet you would refer to it as earth too regardless of its correct name).