More weight does not mean less grip. It means more grip, but the additional grip is not enough to overcome the additional inertia created by the extra weight.
Aerodynamic downforce does not create additional inertia, so it is “free” additional grip from that point of view. With aero the penalty is drag, not inertia
Don’t feel bad about the confusion. For decades this was seriously debated in racing circles, with some contending that more weight helped the car “hug the road” better. The debate ended around the late 1950’s, with Colin Chapman repeatedly beat heavier, higher powered cars with his lightweight designs.