Look, you're still clearly missing the point.

Let's say there's a gym with a lineup of 5 CP 1000 defenders. There are two attacking players. Player A a lineup of 6 CP 500 pokemon, player B has a lineup of 6 CP 2000 pokemon. Just for the sake of argument, assume player A takes 30 seconds to kill a defender while player B takes only 10 seconds to kill a defender.

Now if player B starts attacking the gym, kills the first 2 pokemon and then player A starts attacking the gym, they will never battle the same pokemon at the same time. Player B gets to the 5th pokemon at 40 seconds, and at this time player A is still fighting their first pokemon. So there's absolutely no overlap and there is no "huge upfont damage" for player A

Now reverse the order. Let player A start fighting pokemon, and once they defeat the second defender, player B starts going through the gym.

So at 60 seconds player A is on defender 3 and player B is on defender 1. At 70 seconds player A is still on defender 3 and B has moved onto defender 2. Now at 80 seconds, player A is still chugging away (for 20 seconds) on defender 3 and player B enters into that battle. Player B now receives 20 seconds of upfront damage from that CP 1000 defender. At 90 seconds, both player A and player B defeat defender 3 at the same time.

So just look at what happened. On the 3rd pokemon, player B received 30 seconds worth of damage from the defending CP 1000 pokemon when by definition he really should have only received 10 seconds worth of damage had be been battling alone.

If you think this is "fair" to prevent mooching so be it. But if you just continue that example above player A winds up mooching on the 4th 5th and 6th battles which all only take 10 seconds, because player B storms through them.