The Spanish courts have rejected this, and the most official statements coming out of the EU by Junker, stand by Madrid's position.
Your Wikipedia quote is not relevant in this case. Both legally and pragmatically.
For example: the city of Manchester cannot separate from the UK by referendum.
The city of Montreal, as I mentioned, contemplated this during the Quebec referendum and it was squashed.
By the UN's vague definition, and 'individual' could claim that their 'home' is sovereign.
Also, legally, the UN has no jurisdiction in this affair.
But most pragmatically - the referendum is invalid because it's not being conducted properly. The state police are shutting down polling stations, urging people not to vote, indicating that 'it's invalid' therefore deterring a lot of people from voting.
There is zero legitimacy in this referendum, legally.
Popularly it will be a different thing.