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)