That's actually much too small to be a sand worm of Dune. A baby one, to be sure. The sand worms of Arrakis can eat a harvester in one gulp. A harvester has a crew of almost 30. That thing's mouth isn't large enough for two men. Much less thirty and the giant machine they rode in on.
I think it's likely that the Fremen spent hundreds of thousands of years riding the worms, to the point where they might have shared some sort of psychic link with the worms. The book is very clear that psychic potential exists within the setting, so who's to say that the Fremen don't have some sort of mental link with the worms? If that were the case, the hooks would merely be for holding onto the worm, or possibly a way for the worm to know one of the Fremen were there.
As Uber says, the actual method of maker-riding was left pretty vague. There are many possible explanations as to how the riding is possible, though few of the plausible ones directly involve the hooks as anything but ways of hanging onto the worm.