We kinda already knew that Glossaryck was there from the beginning in order to teach the Butterfly family how to wield the wand so it's not really surprising. What's really surprising is how the truth about how it happened became obscured due to time and a brief cover story told to a bunch of hypnotic suggestion-prone travelers who appear to be in the same state Moon and Star were while in the Realm of Magic.
They were standing in the bog that would eventually house the magic sanctuary which we know is connected to it. How did they get there, and where did they come from? The Realm of Magic seemingly connects to various places around Mewni and even other dimensions, could these early settlers have come from elsewhere in space and time? I've seen theories, possibly due to their particular style of clothing, to be a pack of lost humans from some time in Earth's history. Everything from the missing Roanoke colonists to Bermuda Triangle victims. They arrived on Mewni, dazed and confused, and Glossaryck basically fed them their new history and the means to wield magic before absconding. No idea if that's the case here, though. Though the show already established that there's really not much of a difference between humans and Mewmans.
But yeah, it's kinda weird how casual Glossaryck is about everything, including seemingly instigating the monster rebellion that would eventually get Comet Butterfly killed and cause all kinds of hardships for the kingdom. He's admitted that his only purpose is to teach future Queens of Mewni how to wield magic, so his seemingly exclusive interest in Meteora is telling.