This doesn’t seem to contain any information relevant to the legality of voting. It merely discusses the validity of the referendum, which is a separate issue.
Nothing about (the English translation of) that sentence supports what you keep asserting. If a Spanish high court decided that's what it means in a decision, that's fine, but you should probably link people to the text of their decision instead of repeating the phrase over and over again as if it's magical.
It still certainly doesn’t make it illegal to vote. This constitution merely tells you what’s a valid referendum and what isn’t. It’s not against the law to participate in an “invalid” one.