Explanation from an earlier Wired article:

A communication is ‘testimonial’ only when it reveals the contents of your mind. We can’t invoke the privilege against self-incrimination to prevent the government from collecting biometrics like fingerprints, DNA samples, or voice exemplars. Why? Because the courts have decided that this evidence doesn’t reveal anything you know. It’s not testimonial.

Virginia judge: Police can demand a suspect unlock a phone with a fingerprint | Ars Technica