What are the odds it turns out to be a cassette tape that was spun through the recorder by a hamster on a wheel? Poor little fella. It deserved better.
More seriously, they might only be referring to the support vessel. While that one sentence is a bit ambiguous, every other usage of the word “vessel” is clearly in reference to the support vessel (for example: the support vessel Polar Prince, the Canadian-flagged vessel Polar Prince, etc). While Titan itself seems to be referred to consistently throughout as a submersible, and never a “vessel” (for example: the submersible Titan). In which case, it might be as simple as whatever hard drive stores chat and/or position data. Which wouldn’t be too surprising. It common nowadays for ships to use electronic charts for navigation and such.
Reminds me of that thread about the pitfalls of nuance and whether “vessel” means “canoe” (turns out it does—but I could have told them that for half the price of a fancy Canadian lawyer).