The legend Pirate and Archer

It uses the exact same first job that an Adventurer thief uses. That means that you talk to the Dark Lord to advance to first job. That means the new class is an Adventurer, not a Legend. Legends have their own lands, own job advancements, own code in the data, etc. etc. Everything except the "6 job advancements" points to it being an Adventurer.

Just speculating, but they could do something along the lines of The Dark Lord somehow being the legend, and training you to use his special skills to fight the black mage.