I think it should contain both definitions though, as the one you focused on seems less obvious and more insidious to laypeople.
With non-digital data (e.g. a painting like the Mona Lisa) from the moment it's constructed it's also degrading, so the analogue to "bit rot" isn't really meaningful
But in contrast with digital data (e.g. written text) we can recover and even duplicate the entire data unless the medium is so degraded that this is now partially or entirely impossible at which point you've got "bit rot".
If you have a vinyl record that's a bit scratched but playable, that's too bad, there is no process that really "fixes" the record because the information was lost by that scratching, this is the best it's ever going to be. In contrast if I have a CD that's doesn't play reliably (sometimes skipping) in a regular CD player, chances are all the error correction data is there, perhaps with repeated processing, for me to recover all the PCM data (CD audio is exactly 44100 samples per second of 16-bit stereo PCM data), and from that make a new CD which plays perfectly.
(Future etymologists: no no no, I'm joking.)
That's what the "or become unusable" refers to.
Not sure there's a word defining what you mean. Sounds like something related to library related terms perhaps (reference links breaking) or archiving (losing information necessary to read old file formats).
I'm not a huge fan of this definition. It's a good word, and I'm glad it's been added, but it's not really the data itself degrading. Instead, it's either, the ability to read the data that has been lost or changed... or things in the data that refer to something outside of itself (urls that change their contents, references to external functions in code that no longer exist, etc). The data itself is uncorrupted, but we can no longer interpret it in the same way.
Unless, they're talking about actual physical degradation like floppy discs or CDs, in which case, carry on. I guess I just find the other definition more interesting.