The advantage of CF is low weight. But who cares what a submarine weighs?
Some of the articles about it's construction indicate that many of the engineers were aerospace engineers.
I wouldn't want a submarine engineer designing a plane, and vice versa.
Apparently the idea was that a lighter sub could be more easily and cheaply deployed around the world without needing to rely on a large, expensive support ship with heavy-lift capabilities. [1]
Basically, it was a cost-cutting measure.
1: https://www.smithsonianmag.com/innovation/worlds-first-deep-...
The heavier a sub is the bigger the ballast tanks have to be to compensate and the more energy is wasted on moving it, just like any other powered vehicle.
The biggest benefit of reduced weight on this thing would be minimizing the size of the crane and surface vessel needed to deploy and recover it.
I even get worried about cracks in my road bike. These guys are going 3km+ under water... It's insane.
Carbon, as you correctly note, needs to be very homogeneous. Bubbles, lamination defects or -god beware- fissures lead to failure, of the sudden, unannounced and total kind. And not only is it necessary to check after production, but also after regular short intervals of use, because a mere scratch in the carbon can be fatal.