Subs have dual hulls so they can fill the void between them with water to sink, or air to float, but they don’t dive anywhere near as deep. Most regular naval subs operate above 800 meters. They only have to be below the surface of the water for stealth, not at the bottom of the ocean.

This craft was a single hull design as far as I’m aware, designed with a rigid hull to try and counteract the pressure at the bottom.