I'm pretty sure the issue was with 'move fast and break things' and not using carbon fiber.

I think it was on the youtubes I was watching a story about how they built that thing and it was <spoiler alert> not really fit for purpose. I mean, no big surprise in hindsight.

Carbon fibre has poor compressive strength and good tensile strength.

That makes it inherently bad at holding pressure from outside in a submarine and good at holding pressure inside a spaceship or airplane.