Does this mean the passengers had an instantaneous death?
Absolutely occurred in under 30ms (upper bound). Possibly as quick as 2ms, and in all likelihood well under 10ms.
The depth of the Titanic wreck is 3800m; down there the vessel would be subjected to 380 times that pressure. So...probably.
Mythbusters did an episode showing what happens to a human body under deep sea implosion
> https://youtu.be/LEY3fN4N3D8
It's ugly.
At their depth, where presumably the carbon fibre let go, it's probably a lot faster too. At least they won't be able to even register what's going on.
In a sizable volume of gas the air molecules have a lot of room to bounce around. Moving molecules and them hitting things is heat. So suddenly having a lot less room results in much more frequent impacts. Frequency of impacts is heat, so it becomes hotter. We'd sort of normally consider it the overall average velocity of the molecules, but if they never hit anything they never transfer energy, and aren't measured (or measurable). But when they hit things they transfer some of that energy and it's measured as heat as an aggregate.
So biggish volume of gas, suddenly in a tiny volume: huge spike in heat because all the molecules now slamming into each other and the people inside.
I'd say it would be very unpleasant, but it's so fast and so violent that exceeds the speed of human thought, so they felt nothing and just sort of stopped existing as corporeal beings faster than they could possibly comprehend the change in circumstances. They were. And then they weren't.
Video showing a Mantis Shrimp punching a shell incredibly quickly it creates a vacuum which generates intense heat and a flame .. under water!
Watch how quickly those glass objects imploded as much lower pressures than they would have been at, it would have happened faster than their brains could have comprehended it, so there's some mercy in that at least.