I believe this is a bug, but it could be intended.

Basically you can use sail frames (instead of actual sails or wool) to power a windmill. Considering how sail frames are clearly hollow, I have some trouble believing one could do the same in real life... It also defeats the purpose of having to gather wool etc etc.

Love yor mod, cheers

This is intended actually, has been talked about in the Discord and confirmed by the developers.