it would be a pointless thing to lie about

It was started as a prank against boys who masturbate but don't know much about the world, like the idea that girls can tell you've done it. It's childish and a lot of people keep it going.

I firmly believe the people though that the people who think they can tell are just smelling BO or the person has terrible hygiene and is not cleaning up after doing the deed. (Body/sweat/clothes smells are going to be much, much stronger than any remaining residue from sexual activity, alone or with others.) It's very easy to get a confirmation bias here because I guarantee if you smell someone deliberately you're going to notice smells you don't normally, and if you ask them if they've masturbated that day, most guys if they were honest will say yes because for most guy's that's a daily activity anyway. I would need to see an actual controlled study to think there's any reality to the claim.