I agree, that he could have maybe used other ways of making some points. But the way he filmed the encounter between Marion and the stranger was very much reminiscent of Marion and her husband and the "rape" scene at the beginning. So the beginning of the film shows us again where Marion falls into the trap of her "submissive" behavioural patterns (though she tries to fight it more this time), and the scene "before", deepens our understanding of her character.
I don't think the characters are hollow. They are very introvertive, and unsure about themselves and their feelings. Both have imo what you could call an "inferiority complex", constantly trying to prove themselves, re-determine their image of themselves, etc. 
I would call them immature, not hollow. But why Ozon doesn't give us "more", is probably because he wants to make us think about ourselves, to make it possible to find ourselves in the characters somewhere (I found myself in both quite a lot, actually). If they would be more clearly rendered, everything would seem more determined. It is also only a fragment of their relationship that is shown, though all of the moments are crucial. 
I think it fits more Ozon`s technique as a filmmaker, not to judge but to observe, as well as get the viewer to interact with the film on their own. 
But as you can see from my rating, I don't value it highly. Maybe those grades are worth something in the end