Thank you.
Korriban was never stated to be the homeworld of the Sith species before KOTOR, so if they used the Sith species in place of the Rakata, they could have said that Korriban was just the only colony of the Sith species that the Jedi Exiles knew about and made the tropical planet at the end a colony that was founded before the Korriban colony and where the Sith species still thrives.