I solved the mistery.

It is possible to fit an aero bumper to stock fenders.

The only thing is that you have to take off the fender guides on both sides, because the stock guides keeps the bumper in the position to fit the wider aero arches. Without the guides, you can easily ply the bumper to fit the fender, is not stiff at all, but some other type of guides must be made to keep the bumper in the new position.

Will post pictures after installing.