There's a vast difference between not cooperating and actively impeding. Destruction of inculpatory evidence, for example, is a crime, even if, by some definitions, destroying the evidence could be termed "not cooperating." That's why, IMO, the court makes a distinction between compulsory decryption of a drive which the prosecution already knows to contain incriminating data (which isn't testimonial, and hence not subject to 5th Amendment protection), versus being compelled to decrypt a drive because the government thinks it's likely to find incriminating evidence on it.

Actively impeding the prosecution, in the form of refusing to decrypt a drive known to contain inculpatory evidence, is in many ways tantamount to destroying that evidence. (Assuming, of course, reasonably secure encryption technology, &c.) You're not incriminating yourself by giving them the plaintext; they already know you have it, whether through confession, third party testimony, or some other means.

What the court is saying here, OTOH, is that it would be testimonial, and hence subject to 5th Amendment protection, for you to decrypt a drive the prosecution merely suspects to contain inculpatory evidence, however strong their argument for suspecting such evidence to exist (and they'll have much better arguments than, "Well, drive is big enough to hold millions of files..."). If they were to find any such evidence, they'd have found it through compelling you to provide it to them, and that's the thing they're saying is counter to the 5th Amendment — that, in providing them with evidence they didn't already specifically know to exist, you've incriminated yourself.