Posts are freezing when you are thinking for too long. I post through the Woodalls portal when it does this.
Yes, structural FG is molded as one piece but there are seams and joints. I saw how they make boats in the mold and canopies are probably no different. Boat hulls are in the water, not under UV. When not in the water, they are covered (and often when on water in marina as well).
Another thing that he said - that there are 2 layers - could be a foam filled in between, like they do in boats. Water gets in and stays in.