I haven't done Paladin, so I don't know if I'm missing context, but to me it sounds like they could mean "stolen" in more of a metaphorical way. Or, like. Not so much he went and stole a crystal, but that he left (or was cast out? or what?), and the crystal went with him, so they considered it stolen from them. Or I guess since it's a literal crystal, he may have considered it his, when he left with it, but the Sultansworn didn't?