Ace passes through the passage while impersonating Antonio, especially where the Demonic Winged Tiger is standing guard, using his stealth and twilight traverse ability.
Although he was disguised as Antonio, he still didn't have his memories, so he needed to be very careful about how he interacted with the members of the Skylar Clan. He had already decided that if someone doubted his identity, he wouldn't hesitate to get rid of them.
It was also his first time leaving the underground palace without any need to sneak around. He already knew where the passage was because of the fate map, and he had been there before in Leroy's disguise.
When he approached the hidden passage's exit, he found that it was locked in a formation. After skimming through Antonio's stuff, he found a formation plate, and after using it, the passage was opened.
