Only way I can make sense of this is that Tails somehow got on the Egg Carrier and eavesdropped on Eggman and that's why he tells Sonic that Eggman will go to Kingdom Valley. That doesn't explain why Sonic wasn't around though. Maybe he just didn't know. Or more likely some cutscenes/dialogue just wasn't implemented. But yeah, either way I was right, Tails is alone during that part of the story