No, it has to be him. Not only have the artists left a completely blank spot between the left edge of the screen and the characters, but they have also filled it with a suspicious looking shadow.
If that is not Nibbler, then someone f**ked up. It has to be, no other explanation makes sense. Why would you show the shadow of a chair? Shadows even rarely appear on screen on Futurama, unless they serve some kind of purpose; e.g. like Nibbler's. And when they do, it is either to show some kind of lighting, or some kind of hint (as in this case).
The shadow is shaped like Nibbler, it is, trust me, stare at it enough, and you shall reach the same conclusion. Anything else must be poor sight on your behalf.
What exactly it means? That is what leaves me speculating, but I guess it might have something to do with what happens after the wormhole.