That's not true. They've been interpreted to be testimonial because they are a part of a chain of events that results in the production of incriminating evidence that the prosecution isn't otherwise independently aware of. If the prosecution can't say "he has plans for the bomb on his laptop, I saw a photo of a detonator before he closed the lid", they can't force you to disclose the password to go looking for it.

Hubbell suggests that "being forced to surrender the key to a strongbox" does not violate the Fifth Amendment. Under spdustin's generalization, the strongbox hypothetical would violate the Fifth Amendment.

Hubbell is based on the idea that providing documents responsive to a subpoena is itself a testimonial act, because it is "necessary for [the Defendant] to make extensive use of 'the contents of his own mind' in identifying the hundreds of documents responsive to the requests on the subpoena." I don't see how the purely physical act of unlocking the phone is similar.