Actually, it seems the path to the Duke's Library doesn't necessarily require obtaining the Royal Artifact to be opened.
It's just that after acquiring the Royal Artifact, the Sain Dungeon will provide indications about the existence of the path's location, and before that, this road is obscured by an illusion similar to a hidden door.
Fingerley only found out about this afterward.
Even though adventurers know there are hidden doors within the Sain Dungeon, they wouldn't idly suspect that a random wall in the wide open space of the sunny Royal City could be a hidden door, right? Generally, only in those complex passageways would people start considering the possibility of hidden doors.
The Sain Dungeon uses people's habits against them.
After defeating the Steel Giant Puppet, all Fingerley and his companions had to do was take the Bat Monster's Express to reach Yanor Londo, getting a chance to catch their breath along the way.
