This is a very common phrase. It's often said to children, and it's been used by, for example, feminists when raising awareness about rape culture. "No means no" means "no should be considered absolute the first time it's said, not 'I can be persuaded,' or 'ask me again' or 'do it anyway' or 'yes.'"
He was saying that it was political, because "no means no" is a political slogan against rape. I'd say a silly one because they know they are bad people.
In America feminists and some other people used it as a chant a few times. Which, once more, I think is silly because there's nobody out there who was like, "I was gonna pin down a stranger and ruin their lives for my brief amusement, but I heard a loud chick on my college campus say I have to respect the word no."
I think you're right in that 'No means no' is never going to stop someone who already wants to commit a crime, but it can be used in a more educational way. Not many people who are already in the mindset that they would commit rape will take much from the lesson, but a child/teenager learning the boundaries of consent might. So I agree that it is not as useful in reforming rapists, but it could help in preventing some rapes (Say where two people are getting passionate, but one partner decides they don't want to go further. Learning to accept that first no is very important, even if it might have seemed like your partner was into it a second ago.)
kiwivimt: I wish you were right, but I have found that rarely can such a thing work. We have a national mindset that it is bad, and I think that alone is an amazing thing that some places don't have.