‘Minions’ (in a particular sense… Sometimes it is used in a broader sense, including also mercenaries, which is very confusing) and ‘mercenaries’ are distinct types of units, and some abilities which require a specific type of target, the description usually including a specific wording, do not work on a wrong type.
As a sidenote, the aforementioned ability would have been quite overpowered if it could take over actual mercenaries, breaking all the balance in PvP and ruining also the PvE. I believe there still might be encounters where it could be used, since some bosses have ‘minions’ with them, but I’ve not really looked into it in detail.
I’m not sure if that’s actually the case.