Voting in an election or referendum gives explicit support to the right of that referendum or election to exist, so I don't think your analogy holds.

I think the result should absolutely cause a second, officially sanctioned referendum, for what it's worth.

Those elections weren't ruled unconstitutional with the national government instructing people not to participate. Obviously people choosing to participate were going to be disproportionately those choosing to reject the authority of the national government - i.e. the separatists - even if there hadn't been heavy-handed policing.

A more relevant comparison would be the Scottish independence referendum which managed 85% turnout.

The last unsanctioned Catalan referendum got 80% voting for independence, and that was when opinion polls gave No a huge lead (they seem to be suggesting if a proper vote had been held this time round, Yes to independence could have won, but only narrowly)

Though secession surely would require a higher hurdle than normal parliamentary elections, for such momentous an election? These aren't normal elections,but the breakup of a major European state.