I think they were talking about the papertowels that you use to wipe off fingerprints/hairs/etc from the bathroom. There's no point in trying to hide your crap-covered toilet paper, because you've already dumped your actual crap into the toilet. In general though, no one is going to be sifting through airplane toilets looking for your DNA. The only thing that you might be protecting yourself against is if 'they' are looking for someone else by sifting through the toilet contents, and happen to match your DNA against something they have on file somewhere.