You can look at that a few ways. You are probably more likely to hit someone with the rounds that you miss your intended target with. So if expanding ammo is more lethal then you going to do more damage if you hit someone accidentally.
I have to be honest...I've never seen this argument before, and it defies both logic, and what actually happens in real life. Let's say you're right, you're more likely to miss than hit (though I completely disagree that you're more likely to hit a bystander with a miss than you are to hit your target). Let's assign a 30% hit rate. That's 70% of your rounds that go past your intended backstop with HP ammo. Using FMJ, most hits, are going to go through and through. So now, a number more than 70% are going past your intended backstop. It'll probably be over 90% of your rounds are now going past your backstop. You are responsible for every round you put down range. I'd much rather have 70% of my rounds going down range, than 90%+.
Lets not forget that if you're using JHP ammo, you'll likely need fewer shots to stop an attacker, thereby sending fewer rounds down range.
In Mas Ayoob's book, Gun Digest Book of concealed carry, he breaks this down much better than I can, and he uses real numbers, not made up numbers. The conclusion he draws is that you are less likely to hit/kill bystanders with JHP rounds than FMJ rounds.
There's a reason that basically all professionals and experts in the field of self defense say you should use JHP rounds. They are safer for everyone involved, except for the target of them.