Could also have been a prank played on somebody who wasn't even aware

This presupposes knowing that a fitbit uses bluetooth. As I understand it, there are also models (e.g. Fitbit Charge) which cannot be turned off anyway.

Plus there is an overall assumption here that the owner of a Fitbit knows that the device nickname is visible to anyone, and not just themselves.

These things are certainly not at all obvious in an app-centric bluetooth device context.