Off the top of my head, I’m pretty sure I read it in New Scientist years ago. With some googling, I’ve found lots of people making the same claim in various comment sections, but not anything coming down one way or the other from anywhere especially reputable. I’ve also found some people on a soap-making forum saying that you don’t need preservatives if you’ve made the soap yourself and ensured it’s still alkaline after all the fats have saponified (which won’t necessarily be the case for store-bought soap as they often have extra fats so they moisturise skin a bit).