It was stated that the Observatory in Elysia found Tallon IV to be habitable and did not mention Zebes. So it makes sense that the Chozo were driven to that solar system looking for Tallon IV and at the same time found out about Zebes and colonized too. I say this because it would not make sense for a civilization like the Chozo to build in Zebes, then move to another solar system, colonize Elysia, and build an observatory only to find out that the planet next to theirs is also habitable.