This is like saying there is no mention of the word 'purgatory' in the Bible.
The fact is that a group of cardinals got together when there already was a valid pope, they acted as if he was not valid and elected an anti-pope. To argue that they didn't believe that the See was vacant is possible, but I don't see it as likely. They denied the legitimacy of a validly elected pope. They didn't say we refuse to submit to any successor of St. Peter like the Orthodox do. We can't know their thoughts, but I'll be willing to bet that the majority who elected an anti-pope truly believed that the valid pope was no longer pope. If they weren't schismatics, then they truly believed there was no pope for them to submit to.
I'm not sedevacantist. But I don't believe that sedevacantists in general are schismatic. Conclavism seems to be the real danger with sedevacantism.