Why could God say the Kingdom was prepared for these sheep before the world was created?
Because God, in His foreknowledge, knew exactly which Gentiles would hear the Gospel of the Kingdom from the 144,000 and believe. Their inheritance was not an afterthought but was prepared for them before the world was created, just as Ephesians 1 speaks of being chosen before the foundation of the world.
Les explains that the sheep and goats are only an illustration for the King separating believers from unbelievers. The Kingdom is prepared for the sheep, and Les ties this to Ephesians 1: God chose them before the foundation of the world. In His foreknowledge, God knew exactly which Gentiles would hear the Gospel of the Kingdom from the 144,000 and believe. Therefore He could honestly say the Kingdom was prepared for them before the world was ever created. Les also notes that the works in verse 35 did not save them; they distinguished them as believers.