Now being so extremely fitted, the only way of passing these over the head would be if they open or stretch somehow. So my theory is that these pleats aren't sewn in place, they're only gathered and tied in place, possibly from the inside.
How do I come-up with this conclusin you ask? Well it comes from these other german exemples where there's obviously no waistseam and yet, something's gathering the skirt such as a draw-string front the inside and it's pretty clear that this isn't due to any sort of belt. Check this out...