You can see ‘something’ in that video, and what that something is isnt clear. I was able to screencapture the only frame containing that object in movement. And, that object in movement was the cause of the Pentagon explosion.
I find it hard to make out a 757 in this screen capture. The object is small and hugging the ground. Given its orientation in the video, I dont think that the object was transitioning from higher altitude flight to low level flight as an 757 might be flying. It looks like simply an object that is following the ground.