They do need a warrant now, although they did not used to, and obviously don't need one if you hand an unlocked phone over upon request (same as if you go along with 'let me see your backpack').

It's unlikely this ruling will be upheld since providing biometrics to unlock a phone is difficult to construe as 'testimony' since it's performing an action, like complying with a court order to unlock your front door in response to a warrant should you not desire for the police to kick said door in.

The biggest gray area currently is whether typing in a password is 'something you do' or 'something you know', and you can find different precedent to support both conclusions in US jurisprudence for the time being. Hopefully SCOTUS will come down on the side of 'something you know' if/when such a case ever makes it before them.