It sounds similar, but doesn't sound the same to me. Also how would you determine the similarity allowed? Maybe if we would have such a measure they could use

It sounds similar, but doesn't sound the same to me. Also how would you determine the similarity allowed? Maybe if we would have such a measure they could use that in voice model training to not allow that much similarity to a single voice, but if we don't have an agreed upon value for that than it's a subjective "sounds the same to me" rule then it's hard to follow that. Ok, they can say that don't train on their voice, but it's very likely that a blend of voices from an "allowed" set could produce a very similar voice to his.