That's right, yes. For all their models, Renault does specify a gross train weight that is less than the sum of the GVW and the maximum towload. One can never load a Renault up fully and then still take advantage of its maximum permissible towload. Other manufacturers do have higher payload margins for their cars. Some don't even make any restriction on gross train weight other than what is imposed anyway by the specified max. GVW and max. towload.
That is not to say that a Renault is necessarily any less safe to tow with than any other car. There are a lot more technical factors that play a role than just weight - rear overhang, tyre equipment, suspension rates, height of centre of gravity, type of braking system control, polar moments of inertia of the towcar, to name just a few (leaving aside the obvious human factors that always apply).
On the whole, 4x4's perceived better towing capability is based on weight alone. However, many of the other issues go against them. They often have relatively soft suspensions with more spring travel, high aspect ratio tyres which are also softer and higher centres of gravity. It is not without reason that Mercedes, for example, first introduced a sophisticated electronic trailer stabilising system on the M-Class as standard. It helps to overcome some of the disadvantages that this type of car has although weightwise it would everything going for it.
It's all a much more complex issue. From a concept point of view, vans (MPV's) are better suited to towing than 4x4's, all other things being equal. The ideal towcar would probably be people carrier with lowered suspension and all-wheel-drive.