The judge doesn't need to play guitar. S/he could just summon in court an independent party that can play guitar and ask questions relevant to the case.
Not only that - they wrote at least two songs that referenced how much they hated "Creep" and how it had become a crutch. It's why I find it hard to believe that "Radiohead" are making the infringement claim here and it's much more likely the record label who own the rights to the recording who are making the claim.
