Nomura decides how the lore works to serve his writing, if he wanted to bring back Roxas and Xion without replicas he could've just handwaved it as their hearts being powerful enough to build a body with the help of Sora and Kairi's magic like when she gives him a body in KH1 after being a Heartless.

KH lore is so broad and multifaceted it's easy to revive characters and do anything in theory.