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.