I have noticed that Theobald used to resist mana reduction from LotL minions if he used his skill, but has recently stopped resisting. (I noticed this at a VF tournament.)
Theobald has been silently nerfed and Mitsuko has caught up with it.
Logical consistency isn’t always evident in this game, but once they decided that Xiahou Dun’s minion can dispel Mitsuko, that means they don’t consider a minion’s hit as a special skill. Which means all minion hits should act in the same way.
Just my opinion, but it just sounds like they’re just closing up some of the programming that they left hanging from the original hero. With the increase of minions with special abilities, it’s been pretty clear that they are treated differently from hero specials (just look at Vela and Devana who are immune to minion effects).
Chances are, the case of LotL giving blue heroes minions encountering Mitsuko who already fired wasn’t one that happened often and people may not even have noticed. Now with Waterpipe, they’re just revisiting the old heroes to make sure they act accordingly as their cards say.