> But honestly, there is an expectation of some level of competency/knowledge when trying to "claim" a hobby, because people consider a hobby as something you involve yourself in at more than a surface level.
No. In fact, the very definition of a hobby refutes this.
A hobby is purely anything you have taken interest in for mere pleasure or to fill leisure time. Hobby's synonyms are pastime, leisurely activity, leisurely pursuit, etc. There should be no expectation of mastery.
Now if I said I am a piano practitioner, I would assume some knowledge.
> If someone tells me that they like to play a musical instrument - I'd first gauge [...]
And this list of criteria you gauge will vary from person to person, and interviewer to interviewer, which is exactly why approving/rejecting candidates based on their hobbies is unreliable at best.
By the way, your listed criteria of what someone being a musician is would exclude Jimi Hendrix, Stevie Ray Vaughan, Paul McCartney, Kurt Vile, Tony Williams, Danny Elfman, Eric Clapton and Tom Morello, just to name a few.