The ball didn't stay on that line. It swerves to the left of the post (i.e. over the post/wide).
I've watched it several times back and forth and it's impossible to tell without an horizontal angle.
The camera angle behind the stick is not pointing straight when he initially kicks it. Had it done so and fully zoomed out it would have been a more conclusive view in whether it had or hadn't.
As it zooms out it gives the illusion it's going over however the balls line clearly curves over the post. This is visible on BOTH views.
It's a matter of opinion in terms of at what point the ball goes over the posts. I'm firmly in the it hit the post smack on camp, which IMO is why Bentham ruled that it wasn't over.
There is NOTHING in the rules to state that a drop kick that hits a trajectory of that exactly over the post is or isn't a goal. In that sense, I guess a lot of this comes down to interpretation and the ref interpretation in this case is that if there's reasonable conclusions to suggest the ball would have hit the post and bounced back (if posts are assumed to be infinitely up) then it's not a drop goal. Whereas had the majority of the ball gone inside the post, it would be a drop goal.