Hmm, no - if the gopro had manual controls (shutter speed/aperture/ISO) then he'd simply be increasing the shutter speed (and reducing the aperture) to gain exactly the same result.
If it was darker, say cloudy or dusk - as the gopro has a fixed aperture - it has to either alter ISO or shutter speed to provide the correct exposure, if it used a longer shutter speed, same result. it's why night footage looks crap and grainy normally, much higher ISO which increases the noise on the image.
The point I'm making is, using a different shutter speed or aperture to create an image in the style you want (narrow DOF/blurred background/motion blur etc. etc. etc.) is
just photography. He's not editing the footage that comes out the camera, he's controlling how the light enters the camera to create the image he wants. In a way, yes he's 'editing' the how the image enters the lens and is recorded by the sensor, but that's photography, it's painting with light.
And video is just lots of still images displayed one after the other to give the impression of a moving image, everything that applies to still photography also applies to video.
Saying that he edits the footage is making it sound like he's speeding up/slowing down the footage from the camera, or changing it in some way, when in fact he's just using good photography skills.
Check out his gopro only instagram, he's a very talented photographer, as well as a very good rider
Edit: the new Gopro hero 5 has manual control over ISO and shutter speed, so (up to a point) you wouldn't need an ND filter. However... shooting in bright light you'd probably still need to reduce the light entering the lens in order to get the correct exposure with a slower shutter speed (with an ND filter).