Earlier quoted context omitted.
I believe the debunking is that there's good evidence many of the participants realized they were in an experiment and the other person was an actor, but Milgram hid this in order to get a famous paper.
Does it matter? If someone may or may not be an actor by your judgment is going to cause you to possibly commit an atrocity then you should play it safe and refuse. Just in case...
You'd start by telling yourself that, so it would be easy enough to convince people looking to "debunk" the experiment later.