Fiber composites are complex. Carbon fibers can be made to withstand compressive loads in that you make composites with the CF and other materials. Even so carbon fibers are about half as strong in compression as tension. Even so, carbon fiber might have a specific strength of around 3000 kN-m/kg, steels might be around 63 kN-m/kg. So it's not as simple as "carbon fiber isn't as good in compression as tension, never use it in compression." A lot of current research in aerospace is to produce better manufacturing methods and resins for carbon fibers to phase out aluminum and steel parts. Mostly in tension yes, but compression too, all parts without preloading generally are some degree in compression during their stress lives.

Should he have bought expired carbon fiber for a submarine? No. Is carbon fiber completely absurd in submarine usage? I don't imagine so. Though steel is plenty fine for a submarine, normally the hard part is the sinking by design, not the floating, so weight savings aren't super important.

First, he was an aerospace guy and several things he's said make me think he was sort of chauvinistic about deep sea exploration in general, stuff like "It’s perfectly fine. Having all these certifications for airplanes is one thing, but the carbon fiber was perfectly sound."

Second, his business model, taking four people down with him in something other than Cameronesque claustrophia, and doing so without the cost of owning a proper launch vessel, instead renting any ship that could hold and then monitor his launch sled, meant it was critical he make something big and light, by deep sea submersible standards, that was at least nominally expected to handle the load. Shit, I guess in some sense, he did, since it went down and back two or three times or whatever. At the absolute best, though, he'd invented a disposable sub, and he clearly didn't worry about that limitation any more than the rest.