Onesies and umbilical cord

Hi everyone I am 7 weeks and have no experienc with babies. Was watching a video on YouTube by "Cloud Mom" who I find extremely helpful. She suggested avoiding newborn size onesies because of umbilical cord and said she personally thinks it is safer to dress baby in the side-snap t shirts which don't cover the cord until cord falls off, after which you can put them in size 0-3 onesies. I'm just wondering what would be the real worry if I did put my baby in a newborn size onesie like one time for pictures. Is she saying it may make the cord fall off prematurely and that's dangerous?