Actually, it's not a misconception...
The expert "forgetfulness" affects all 100% of the monsters in all enemy stacks...
The advanced version seems to affect 100% of one stack.
The basic version affects 50% of one stack.
I think that's how it works!
And both you and he are right in your own way: Zhenya meant that it doesn't affect, for example, titans or liches...
And doesn't the basic version give 50%?