Carbon fiber composites have huge positives and and some really bad negatives. Hulls for deep seas submersibles cannot take advantage of the positives of composite construction and are uniquely impacted by the negatives.
Furthermore, it looks like the composite section of the vessel used a technique known as wet winding which is totally inappropriate for this application due to lack of control of the process and susceptibility to defects that lead to critical safety problems.
And nobody in their right mind uses a cylindrical vessel for deep sea work, everyone uses spherical vessels .
Video showing the sub construction.
Hoop wound carbon fiber and glued to the titanium endcap.
I have seen a video of it being wound. Five inches thick just like ribbon onto a bobbin. If the structure was going to hold high pressure inside a cylinder it might have made some sense, but compressive strength — nah.