As an esoteric exercise I ran a Boyle's law calculation on the change in volume of the air in the sub at implosion
Assuming the inside of the sub was 5 feet in diameter and 15 feet long the interior volume would have been 785 cubic feet. Pressure in the sub was 14.7 psi ( normal atmospheric pressure).
If implosion depth was 11,500 feet, sea water pressure was about 5100 psi.
At the instant of implosion the internal air volume (785 cu/ft @14.7 psi) would have been compressed to 2.8 cubic feet at 5500 psi.
Not a lot of room for 5 bodies in 2.8 cubic feet. That is equal to a box about 17" x 17" x 17".
It is even worst. Your calculation is quasi-static.
And unnecessary complex. If external pressure at 4000m depth is 400 bars, the internal volume will become 1/400 of the original volume.
But the event was highly dynamic...
The shell parts were accelerated towards the center, were air and corpses were compressed. Due to inertia, the compression reaches a peak pressure several times larger than the external pressure, so in the maximum compression instant the inner pressure has peaked somewhere around 1000 bars (2.5 times the external pressure of 400 bars).
Perhaps even more.
So the inner volume of the shell was instantaneusly reduced to 1/1000 of the original value.
But as the compressed material is elastic, after reaching that very high pressure peak, the compressed material did expand back, projecting small chunks of the shell and its contents all around.
Only very hard parts, such as the titanium domes, could have survived the secondary explosion.