I see your point, but honestly I really think that’s not quite definitive in this particular situation. Sylvanas figured she had a shot against either one alone (which I doubt), so you’re correct on that. The problem is that in the book we didn’t see a fight, what we did see was Malfurion taking a hit, a good hit from Sylvanas, and being who she is she could definitely get some got shots into either one during a fight.
Then Blizz chose to completely contradict that by showing that Malfurion actually beat her. The only thing that is present in both versions is that Saurfang ended the fight. Normaly I would argue game events over book events but this is one of the few if not only situations where we get to experience an current event that takes place both in the book and the game. Not to mention one of those (the book) didn’t give us a full view of the event in question.