First time I ever heard this. All of the characters believed it was Ethan who killed Scott. Ethan also left a note saying he would kill an additional person for each day that they held off in handing Claire back to him. There was never any hint that it might have been the monster.
And the episodes dealing with Ethan definitely inferred that he had abnormal strength. That's why he was able to get up after Jack gave him a pounding and why Sawyer said (right before Charlie shot him) "I want to know who or what this guy is!"
Another interesting clue about this is an interview I read with Michael Emerson (who plays Henry/Ben). Emerson said that in the scene from the episode last fall when Ben leads Sawyer up the mountain (to show him that they were on a separate island), while filming the scene, the director kept stressing to him to look relaxed like he was having an easy time scaling the hill, while Sawyer on the other hand was clearly exhausted and out of breath.

