The point is not that there is no slipstream overtaking, but that there is literally (<5ms per longest straights) zero slipstream. When you say there is no slipstream overtakes IRL, it means nobody drafts by someone else. But overtakes are facilitated by the fact that a big car at 250kph leaves enough slipstream that if the car behind is 3 tenths behind exiting last corner at Bahrain, it will be 0.1 or 0.150 behind going into the braking zone and therefore being able do initiate a move in the braking zone.
With rF2 Porsche cup car, you exit corner with 0.2 gap, and after 1km straight you are still 0.2 behind. This is not how physics works, unless Porsche has managed to create an aerodynamics miracle (they haven't).