This is really not a useful argument. Self determination is a natural right. The idea that it's against the law to seek it doesn't work when the people themselves are arguing they don't want to be under that law.
Then you should use the process laid by the constitution to change it, not unilaterally decide the law doesn't apply to you.
This argument seems a little bunk to me. Certainly my nation (US) wouldn't exist if they followed this logic.
It wouldn’t exist either if unionists had not fought and won a war against secessionists.
But then you can apply same logic when a city or a village vote to not pay taxes there, or have special rules, my opinion is that constitution is a very important law and there is a reason that changing it requires the majority to vote(probably differs by country).
I understand your point but to get the independence is something extreme in this case it should not be a simple process but something more complex otherwise bypassing the constitution can happen every month.