It is not true because named unique characters are not allowed to have other command traits and artefacts, not command ability. In cases of some unique character(as Aventis), their sub-faction is fixed so cannot take other sub-faction's command ability. But if a unique character have no fixed sub-faction, it will be chosen sub-faction of the army during the game.
Therefore, you can use 'Leave None Alive' with Skarbrand.