Roberts' answer is that it depends on the particulars. Congress controls the purse strings, Roberts pointed out, so it can defund a war. Just as a curious Constitutional question, Leahy's question is interesting. I wish Roberts had a better answer. One would think that Congress does not give a president the authority to wage war in perpetuity. Indeed, Roberts might have mentioned the Revolutionary War, where Congress was poised to quit fighting and negotiate with England several times. Washington was always able to talk them out of it. That history (and I realize it predates the Constitution) perhaps gives us a lesson.
All that said, Leahy is still a tool, though he's not being as big a schumck as I expected him to be. Kennedy and Biden, I trust, will not disappoint.
How could he answer it? Congress certainly has the power to pass a law ending a war. They can override a presidential veto.
But if that ever happened, it would have to go to the Supreme Court, where lawyers for the President, and lawyers for congress, would both spend thousands of hours researching the issue in detail, so they could present written briefs.
Then the supreme court would have oral arguments, where the lawyers would answer questions. And then the judges would do their own research, and talk to their fellow justices, and make a ruling.
How could Roberts possibly answer how he thinks about the question NOW? He can't say how he would rule in that case, or any case, not just because it is wrong, but because it would be silly to do so before having all the information.