I'm with you on this one.

Virtually all of the instructions on the site begin with folding the paper in half in some way. However, this one breaks that rule:

https://www.foldnfly.com/15.html#Spin-Plane "1. Fold the paper in half about two inches before the edge."

But it doesn't have to! It could just as well have started with a center fold and then folding the corners to the center to measure, then folding that over to measure again. The measurement folds don't have to even be full folds -- you could just fold only at the very edges to lock in the measurement. Also, this plane ultimately has a center fold in it, so it could easily have begun with one.