Here's the relevant passage:
Of course, even if the government is only barred from using the evidence in the subject's own trial, this may still be quite troubling. It's one thing to say "Well, it's true that this [burglar or robber, or even rapist or murderer] will have to go free even though he's clearly guilty, but that's what we need to do to give the police an incentive to behave properly"; it maybe another to say with respect to an enemy saboteur who may have special skills, willpower, and connections that might allow him to fight against us again. But at least this consequence is less troubling than a conclusion that the government was simply barred from interrogating the enemy in various ways in the first place.
It's telling that he used the words "or even rapist or murderer" instead of making the direct comparison between, say, an enemy saboteur and an arsonist. The relevant question is where the difference lies- assuming the arsonist is destroying buildings because he hates the government (which is possible), where is the distinction made between a saboteur and an arsonist in terms of their civil and legal rights, if the damage is the same? I hate to bring up such a loaded term, but it seems that the difference is purely political (war is, after all, politics by other means), making the distinction rest solely on the former's status as a political prisoner. Volokh is worried about how troubling it would be if the police couldn't interrogate a prisoner, but let us remember that dead is dead, whether at the hands of a random criminal or an enemy of the state. Arguments that the rights of one should be ignored raise the question of whether the rights of the other should be ignored, and Volokh seems to be dancing around that all-important question.