Analysis on what happens when a submarine implodes
twitter.com
twitter.com
Here is a steel pressure tank imploding. It was under only 14.5ish psi. The Titan would have experienced HUNDREDS of psi. Also, it's pressure hull was made of Carbon Fiber, which is notorious for failing catastrophically with zero warning, whether due to a manufacturing defect like a simple nick, or just going beyond what it could physically withstand.
The vessel was NEVER tested to 4000 m. It was probable that it was always going to fail at the depth needed for the titanic. There was no warning for the crew and passengers. One instant they were descending just fine, the next they are human bisque.
Around 6,000 actually, according to Google.
(Not to diminish from the point that from everything we've heard from people like James Cameron, etc, this thing was nowhere near adequately tested.)
Not sure where you found that, according to this article it was on it's 3 trip, although both the previous ones were near-misses:
https://www.nbcnews.com/news/us-news/titan-passengers-share-...
I'd assume the submarine is basically a crashed can now lying somewhere in the seabed, with all its contents still inside, equally compressed.