do you have enough thickness. you could be seeing the back faces of the underside of the wings. If I remember correctly grant even mentions this in the video.
Probably your wing are:
- Too thin, the upper face lies below the -under- face. Showing the red side.
- The upper face is just missing (due too merge by distance?) and showing the inner face
As has been said, especially as there is one patch of Z fighting, it is highly probable we are seeing the back, red, of the ‘front’ faces poking through at the back. A wing membrane thickness issue.