I'm interested in knowing:how is citizenship a legal fiction ?
Most people also associate citizenship with right of abode, but that's not always true either. For instance, the BN(O)s from Hong Kong are British Citizens who on that basis cannot live in England or Hong Kong - or anywhere. A separate document, an HKID or a visa, are required for them to live in HK and the UK respectively.
At the end of the day it's just paperwork that decides who negotiates your right to travel and is responsible for coming to get you if things go bad.
It doesn't really mean much else. Where you live and the government you engage with regularly in my opinion has far more bearing on you, your obligations and your benefits.