The only thing about that is that the submersible has been down there a handful of times, up to five depending on which report you read. Unless something compromised it (e.g. like the shuttle heat tile malfunction) you wouldn't expect it to implode.
A read that the vessel had seven different ascent systems that could operate independently and are considered reliable - so perhaps it is incapable of ascending because it is trapped either at the Titanic wreckage or even in some kind of abandoned fishing net.
I'm also dubious of them being in the wreckage because that would mean they got to it - as I understand the navigation, it requires coms to the surface to guide it to the wreck, and that's part of what was lost fairly early on.