I still don't see how a fingerprint lock vs a PIN differ in this respect. Either way, the end result is the police gaining access to material evidence on your phone. Giving or inputting a PIN is not providing testimony. It's simply unlocking material evidence the same way your fingerprint would.

This is a matter of opinion and viewpoint, but the law doesn't just concern itself with end results. There are courts that don't hold passwords to be testimonial evidence, though (in the US) most do. To the first part (about the difference between material and testimonial evidence in principle, if not practical effect), this goes back to the original causes for the establishment of a right against self-incrimination: criminals had been forced to testify against themselves, even in ways which the general public found objectionable. As to the second part, of whether or not a password constitutes testimony, or is merely an instrumentality towards obtaining material evidence, there is definitely room for controversy, but I would argue that it has that character of testimony which is objectionable to coerce from a defendant.

Does "can be opened" mean they are going to unlock the phone with a backdoor, or that they're legally allowed to force your thumb onto the screen/punish you if you refuse?

I did mean 'can' in terms of 'legally possible', although I would imagine that mobile phone fingerprint scanners aren't really the most secure locks either. The difference between obtaining information from someone's body again comes down to the legal (ethical, philosophical etc.) difference between material evidence and testimonial evidence--it's one thing for me to take known evidence and open it (including using keys or codes that I may have physical copies of, or otherwise obtain through legal means), and another thing entirely for me to coerce you to speak against your own interest.

I haven't read the cases in a long time, so I'm not sure precisely what language the courts use to describe the way they view passwords (codes, etc.), but their 'special protection' (if you view it that way) comes not from the practical effect of opening a locked device, but from the psychic protection of the internal sanctity of the mind afforded by the fifth amendment.