Remember the ENT episode Twilight. Where NX-01 and over 6000 humans settled on Ceti Alpha V. So even 100 years before "Space Seed". Earth Starfleet knew of the Ceti Alpha system.
While in a real world context. This is addition of an event into a gray area because it's a prequel series decades after Space Seed and TWOK. However in the context of the Star Trek Universe, Starfleet always knew about the Ceti Alpha system.