They can’t use foam for flotation at that depth. There is a concrete like “synthetic foam” if you want unpowered rise to surface but it’s expensive. I read early on they wanted carbon to be light enough to get back up by releasing weights on powerloss.
From what I have seen of the hull, it was a tube. I can’t remeber seeing a view with a complex or even drafted shape to it.
I suspect that spinning carbon by fiber would take far too long, and think they probably used narrow weave to do it. It would have been cheaper than the required thickness of titanium, which is the same strength as steel, but 40% the weight and still a little heavier than aluminum.
Who knows!? Nothing they did showed that they ever chose the more expensive option. Carbon was likely just the cheapest thing they could use for their goals.