Saying Nick Gilliard is a valid authority on saber levels by virtue of being a fight choreographer is like saying the small asian who animates Sidious' lightning is a valid authority on Sidious' lightning level or Force power. It's just his opinion and in the end he does whatever GL tells him to do.
Nick Gilliard has also expressedly either disavowed EU forms from Fightsaber etc. or simply remained ignorant to them.
Neph, Grievous has magnetic heels. Even if we assume he is being tugged, (and there's not strong enough evidence to assume Obi would do a weak tug anyways) resisting by virtue of magnetic feet isn't a strong indicator of anything but having... magnetic feet. If Obi fights Grievous on wood floors or a sand arena, that's moot anyways and he can't defend himself by virtue of being a 1337 muggle.
Back to the match, I still reassert that Obi-Wan is too much for Grevous. The general may have gotten some swipes in but when Obi brings Force to bear he just loses it and runs. I imagine even Anakin could defeat him or similarly use TK to chase him off.
And since the asian tech stereotype guy needs to determine how bright, large, loud and persistent Sids' lightning is, the same logic applies.
An authotitative figure is GL in this case, since he leans over in his director chair and tells Nick what happens next. Nick's job is just to make it fast, fun and safe.