Do include one thing that isn’t relevant, they need to somewhat know your personality and they don’t like people that tries way too hard to make their personal statement only about their course. Yea most of it needs to be about the course ofc, but they want to know what u also do in ur spare time (so eg could do something about volunteering, and a sentence about the self teaching instrument). My sixth form teacher showed my personal statement few years back to someone who works with Cambridge admissions (i wasn’t applying there so it was fine), and he liked how I was including that I was learning languages in my spare time and sports etc. I did link what skills it gave me to my course and how it would help (study pharmacy). So maybe with the self teaching an instrument u could say what skill it gives u and link to how u also need that skill in computer science.
Don’t only include “relevant” stuff, mentioning stuff that is not relevant like the instrument, volunteering, sports or anything and say what skills they’ve given u and link it to ur course is completely fine and they prefer that much more than a statement full of only CS related stuff. But yeah still include something CS related ofc as that is what u are applying for. But don’t do the mistake of only speaking about CS related stuff