Inner Furnace - another lie our manuals have told?

[BesucherXia]

I knew it from the manual since long before that this spell must penetrate MR to work, but today I also found by chance that all indy units are candidates to this spell's effect as well.

Luckily that was not a decisive battle using hundreds of archers guarding some VIPs but a small SP game. But I still found it very odd and a great hamper for its use. Would you think it another bug or just throw our manuals away?
(Hmmm... I mean if you don't like your manuals any more, feel free to send it to me.)