Maybe I do not understand it fully (ne pas parle de francais), but where do they game the system?

You buy cheap when you have storage and sell expensive when demand goes up. Pretty much how the market should work?

This way there is incentive to balance it out (by creating storage) - and in this case stabilize the grid.

When the grid would become more flexible, so even homeowners can do this with their solar panels and big batteries - then this should be net gain for everyone, no?

Unless gaming it, with big money, destabilises it. So some safeguards probably should remain.