Did you check the damage routine and check to see which value makes a modification to the character's attack? I would imagine that the routine would checkfor Black Belt or Master, then branch to a routine that would check for equipment and penalize them accordingly.
Best thing to do is compare your rom with a fresh rom and see what was changed, and if any of it was a factor in the Thief being broken.
But what if I didn't even edit that? Does FFHackster edit that? Do you have an offset list for FF1? I think the ones I have are old.