I don't think the intent of the Flattening spell was to give one a bonus in combat. Being 2 dimensional seems like an awkward thing to be already, let alone in combat. And since it would seem absurd to put any rationale behind the topic, all we have to go on is intent. And judging by the other spells on the list, and the name of the list, "Escapes", it makes sense that its intent was to avoid confrontation, not enhance it.
How do you even affect anything when you're 2 dimensional? That's like us trying to touch time. And if you're literally 2 dimensional, I don't even know if you could be sliced through. At least not from a perfectly parallel frontal attack. Yeah that's silly. IMHO, it's just a goofy but cool spell and should be treated as such.