The question that clarifies the matter is not whether something is illegal. You can craft language to make lots of things illegal, especially within a hierarchy of national and regional governments. The question is whether voting for independence is a crime. When you have to make voting a crime, and deploy police to prevent the population turning out to vote, it's a pretty good indicator you are on the wrong side of history.

This is probably not the the way the national government wanted their actions to be seen, nor is overwhelming popular disobedience the outcome they desired.