From the article: He can't be compelled to provide a password if the court can't prove that he is the owner of the drive. The work to decrypt one of the drives

From the article: He can't be compelled to provide a password if the court can't prove that he is the owner of the drive. The work to decrypt one of the drives was done to prove that the drives were his property. If the court is convinced the drives are his property, they can compel him to release the contents of the property. I imagine that: 1) If he doesn't know the password, he would have the first admit ownership of the drive -- this would allow the court to use evidence from the other drive against him. 2) If he doesn't own the drives, or maintains that he does not own them, the court has to rely on forensic evidence they can gather. Their case would be much stronger with more evidence and proof that the drives belong to him.