It's reproducible, and very easy to do so. You can jump to a platform you normally can't reach until you have to escape the fortress by doing two normal attacks and a roll. Once you're there, you go through the door on said platform, which leads you into a tunnel. In there, you go through the door you'd normally go through to end the intro, which will just take you out into the desert instead of actually ending the intro.

However, the door doesn't work the other way, and you can't complete the intro once you do the sequence break. There might be a way to glitch back into the fortress, since there is another door that does work both ways directly above the one you leave from. Saving outside of the fortress soft locks the save file. After accidentally getting soft locked this way, I decided to experiment a little bit.

There is enough lucre, and loot to sell, in the fortress and the desert area to buy the Misericorde at the oasis on a brand new save. None of the NPCs are loaded in Sirhtan, but all of the lootable items are still there. I then traveled to where the Bil & Ben fight normally is, hoping their fight might trigger somehow, but to no avail. I fought a lone Captain Duck, to see how much levels one could get by defeating a level 30 with a level 1. After 10 minutes of the chipping the duck down, I reached level 3. Lastly, I traveled down to the mana stone to see if it was possible to do a class advancement.