Of course Kaiba believes he can get to Atem.
All of the crazy stuff he’s seen. Souls torn from living bodies, an ancient city as a game board, realms where monsters are real with swords stuck in them, ghosts living in gold acting like parasites that can take over their host entirely.
Is the impossible only supposed to be achievable when it’s Yugi doing it? Knowing everything he knows now, Atem should just be in a different world. And dimensional travel is a party trick. Why would it be out of his reach?
Of course he thinks he can.