Even if there were DMZs in the galaxy that separated alien species, it seems unlikely that we'd conveniently be located within one.
This is actually a fallacious argument.
It seems highly unlikely that there would be a monkey in your house. However, if you see a monkey in your house, it doesn't follow from that that, since it seems unlikely that there would be a monkey in your house, it is more likely to be a complicated optical illusion.
Given that you see a monkey in your house, and you're not on drugs, a monkey probably escaped from a zoo and really is in your house.
In other words, the likelihood of being "conveniently located" in a DMZ is totally irrelevant: we don't see any alien empires.
Given that we don't see any alien empires, if you believe there should be alien empires, there
has to be some explanation, however unlikely it might seem in isolation, why we don't see them.
I happen to agree that I think it's most likely that there just aren't any alien empires, but plenty of people believe (on extremely dubious reasoning) that there should be, and we have to treat the probability of explanations from the basis of the facts on the ground, not what we think the facts would be a priori.
To put it another way: If we
knew for a fact that there were such DMZs, and we didn't see any aliens, the a priori likelihood that we would conveniently be located within one of those DMZs doesn't change - they take up the same amount of space regardless of whether they are hypothetical or proven and we have the same chance of randomly choosing that space if we threw a dart at the universe - but we would absolutely be justified concluding we were in one, because the
posterior probability, as mathematicians call it,
given that we see no aliens, is higher.