They don't actually need your passphrase to unlock your phone - they just need somebody with the passphrase to unlock in for them. And if there's any doubt about who that is, then having that passphrase counts as testimonial; but if there's not - it might not count as testimonial.

Although there are apparently a whole bunch of legal details that matter here; courts have in some cases held that defendants can be forced to decrypt a device when the mere act of being able to decrypt it is itself a foregone conclusion.

(If you want to google a few of these cases, the all writs act is a decent keyword to include in the search).

The defendant never needs to divulge the passphrase - they simply need to provide a decrypted laptop.