Its to lower the risk of aspiration (vomit being inhaled - can cause pneumonia and even death) if the woman needs a c-section than poop on the baby. When laying flat or being put under anaesthetic there's a higher risk of aspiration for pregnant woman than the regular person as pregnancy hormones relax everything, including the sphincter keeping food in the stomach. It's the same idea as being fasted before surgery. I'm a nurse and studying to be a midwife and to my knowledge, it's never had anything to do with pooping on the baby. I do remember while at uni seeing and reading some studies investigating the role of vaginal flora on inoculating babies' gut biomes, so it is believed to contribute, but breast milk absolutely does provide a good amount as well, you're right.