The Reason I think people get the idea that billet is stronger, is from steel parts...
Steel that forges well is not the strongest, so a billet part that uses a stronger steel, will be stronger.
With AR receivers, they are all made of aluminum, they should be made of 70 series T6. Since the material is the same, the forging process actually makes the part stronger than billet.
Basically, billet is stronger/better if the manufacturer chooses a stronger material than those that work well with the forging method. But if the material is the same, the forging process actually work hardens the metal and makes it stronger.