The jurors don't have to have "irrefutable proof," just guilt beyond a "reasonable doubt." "Irrefutable proof" would be a video tape of the murder and a confession. Thank goodness juries in the country aren't held to your standards... there'd be even more murderers walking the streets. No, instead, juries are allowed to use common sense as well.
I guess the physical proof of the bodies being at the alibi site, the hundreds of lies that the judge told the jury they could use toward a "guilty conscience," the circumstantial evidence and the possible motive(s) were more than enough for twelve "reasonable" people to override the theory of the "Satanic, homeless band of killers" that Geragos said tried to "frame" Scott Peterson for some unknown reason.
Imagine that... convicting a murderer without the crime being caught on videotape.