Yes because external storage is much larger, and theres nothing more annoying than being in the middle of doing some science with 30 other bits of complex equipment, and then the camera stops working with storage full errors and youre 7000m underwater in a cramped sub trying to navigate a camera UI to find the setting.
Configure your systems so they are in the configuration that is less likely to cause random disruption in the field.
The camera was an off the shelf part (from a very specialty shelf I suppose). It had an SD card built in because some people might not have a thing to stream to; it's probably good for demos and cheap enough to be good for a bullet point. Given the rest of the components inside they probably had enough margin that they weren't optimizing for costs. The value add was in the pressure vessel, and that seems to have mostly worked.
They probably used it for testing only, hence why it had irrelevant footage.
They might have forgot to remove or just didn't care.
Which makes me wonder why they bother with the SD card at all. What was it meant to be storing? If it is not intended to be the real storage area, why not just have it in a loop, constantly over-writing the oldest material?