This describes the mechanism with immediate cord clamping/cutting (as commonly practiced today). But if the cord is not clamped immediately, but left until cord stops pulsating (standard practice before 1970s/80s) there is a period in which oxygen is provided both from placenta and respiration. I would suggest that this is closer to the natural physiology of respiration/circulation in fetal transition to extrauterine life. See Mercer and Skovgaard "Fetal to neonatal transition" in Soo Downe "Normal Childbirth: evidence and debate"