I suppose the article meant cap itself was a carbon fiber/epoxy build, implying nothing about the connection?

On a separate note, although the submarine failed catastrophically, that doesn't mean every decision it took was wrong. I wonder if the bolts actually make sense under high water pressure.

They use some sort of glue to seal the titanium end caps to the CF hull, then the Titanium hemispheres are bolted to the end caps. Here’s their own YT vid showing the construction process: https://youtu.be/WK99kBS1AfE

I believe it’s both really. My understanding is the end caps are bolted to a titanium flange, not the carbon fibre hull directly. The titanium flange the caps are bolted to is epoxied to the hull.

Yea, you generally try to avoid drilling holes or even casting threaded bosses into carbon or other composites, especially this extreme pressure rig, because it really reduces the strength versus an strong adhesive that evenly applies pressure without breaking/disrupting the composite material.

Not really. There were titanium rings at each end of the cylinder, fixed with glue, and then the caps were bolted onto said rings.