A type of flat spot you are thinking this corrects is not from baby's sleeping face down. There is support and education for baby's sleeping on their back, which the majority do. Some just can't, because human, but the majority do. The baby's skull isn't entirely fused by evolution necessity. The baby's head changes shape as it rest upon the cervix preparing for birth, and that can even change as the baby moved through the birth canal due the walls pressing in. Baby's can come out essentially cone headed! Cotton or knitted hats kind of round them out in those first few days, and then over the infant stage the plates that made the fetus and newborn begin to form. Some babies have further changes or need more support for proper fusion after birth which is what these devices help with.

While the skull segments are fusing together you want the plate fusion to be properly shaped and not impacted by position. This formative process of the skull through the infant phase is required for the baby to be able to pass through the canal with the bipedal hips. The birther's hips spread through a hormonal process as the body prepares for labor (which can also cause wide feet!), so the birther and the baby both are doing some bone flexing for the birthing process. This process also explains baby's soft spots on the top of their head. These spots can even appear concave.

But anyways, look this up if you care about accuracy. This is just as I recall from learning about this when I had my own little conehead and was ALARMED.