The hydrocarbon vapours are the crew, so yes an electric sub does have hydrocarbon vapours during an implosion.

At 5,500 psi air and carbohydrates will auto-ignite and flash. The massive implosion triggers an explosion.

It’s not the heat from the air that burns, the pressure simply takes all of the chemicals to a different phase of state. Everything just vaporises and oxidises.

There are real nuclear fusion reactors that achieve fusion by collapsing little bubbles.

Collapse 4m3 of air and people from 15psi to 5,500psi and you will easily go through the auto ignition state and explode the contents of the chamber at the tail of the implosion.

Nobody onboard would have noticed the implosion.