“Why have you been talking to Marv for one hour? Dinner is ready!”
“Not now! We are doing *research*!” https://twitter.com/numidianprime/status/1332461890090512384

Marv mentions this in the mentions, but Tython entered the EU through the RPG. No, not WEG! The d20 version from Wizards of the Coast! It was first mentioned as a possible Jedi birthplace in the Power of the Jedi Sourcebook.

It was the sister book to the better-known Dark Side Sourcebook. Both clarified the Jedi and SIth timelines for the first time ever.

JD Wiker et al decided to "canonize" multiple worlds and concepts from the early drafts as "rumored" or "legendary" places.

What worlds and concepts? Oh, so many. Ondos, Had Abaddon, the Dai Bendu, the Chatos Academy, the Legions of Lettow. Even Utapau, as this book came out before ROTS!

Years later, the New Essential Chronology picked one of all these rumored Jedi birthplaces as the real one: Tython. It would then appear on the Dawn of the Jedi comic and the MMORPG The Old Republic. Even the new continuity mentions it as an ancient Jedi world.

And now you know.