"Add-to-Cart-URL" and Product SEF name

The url used to add an item to the cart does not use the configuration option "Product SEF name" into account.
No matter what I insert there, the url always is like

{prefix}/product/updatecart/tmpl-component{query-options}

where "product" should be the value of the "Product SEF name".

That's normal. The updatecart url does not use that option. That option is only used for the product page URL which is the one that matters for SEF.
In most cases the updatecart url is not even displayed anywhere.