Actually, there is still a slight speed advantage for the heavier rider as the bicycle weight needs to be added. As the bike weight is constant it is more of a penalty for the lighter rider. For a 3.2w/kg 60kg rider an 8kg bike means their effective w/kg after adding the bike weight is 2.82 while for a 3.2w/kg 80kg rider their effective w/kg is 2.91w/kg.
Zwift Insider has done the testing on this and proven that heavier beats lighter in every situation.