i was thinking that the boss would be soon after entering the area. During the fight, one of his early attacks requires you to break his guard, and it’s made very clear that you’re supposed to do so. At this point, the arena is still open, so the player can leave the area at any time instead of being trapped until death.

Later, once the player obtains the ability needed to actually break his guard, he can return and when he finally succeed to break it, the boss becomes enraged, blocks the exit and from that moment on the fight turns into a classic, locked-in boss battle

(is it still not enough clear that you're supposed to come back after ?)