Just imagine if it was an official battle. It'd have still most probably called a knockout by the referee. After all it was Oricorio's attack which send Rowlet flying into Mallow. If Rowlet didn't hit Mallow's head, it'd have obviously hit the ground. How can you say there would have been a massive difference?
I'll just say the same thing here. Just imagine if it was an official battle. Do you really think that the referee wouldn't have called it a knockout?
Realistically if it was a official battle where there was referee then Mallow wouldn't even be that close to the battlefield to where Rowlet being sent flying would whack into her head meaning that, it hitting Mallow's head wouldn't have happened. As for how can it be said that there is a massive difference that's easy it's because no matter how you look at it Mallow is not considered a part of the battlefield where as the walls and grounds are.
It's not rocket science, it's not that hard to understand that Mallow is not part of the battlefield.