Yes, mercs with battlecries do not count, but their abilities with battlecries do.
The easiest one to farm this weekly quest with is probably Drek’Thar.
As for whether this behaviour is a bug or not, I cannot say definitively — however, I do confirm your observations, as already said.
PS
Did you know about the tricks of completing daily quests in Mercenaries? See these, for example: Blizzard can you please STOP FORCING Mercenaries! - #8 by SparkyElf-2852, Questing Emote Suggestion - #11 by SparkyElf-2852 (and the video linked there).