out of stock checkout not working

Hi, I have a shop on OC 2.1.0.1 here : http://test.vente-pieces-agricoles.com.
Checkout out of stock is set to yes in settings, but when I want to add a product that is out of stock I have the alert message and product not add to cart.
You can try with this product that is out of stock : http://test.vente-pieces-agricoles.com/ ... t_id=31653
Any idea ?
thank you for your help