How can disclosing some data that is a function of information you have on your mind not be testimony?

Because it isn't revealing information that only exists in your mind. Keep in mind the reason the 5th Amendment exists: the prevent the government from having an incentive to coerce false testimony through torture.

An evil government could coerce someone to falsely say "I did it!"

An evil government cannot coerce someone to falsely type a password into a terminal to decrypt files with incriminating evidence. Because if it's false that evidence simply won't exist.

> Keep in mind the reason the 5th Amendment exists: the prevent the government from having an incentive to coerce false testimony through torture.

Thats not the sole purpose of the self-incrimination protection (which is, also, far too focussed in it's protections to meaningfully effect that end, since historically false testimony coerced through torture was very often sought from people other than the person it was used against.)

> An evil government cannot coerce someone to falsely type a password into a terminal to decrypt files with incriminating evidence.

Sure they can, or, rather, if they claim to know already what is on the drive and reject any decryption which does not match their claimed knowledge, they can punish someone for non-compliance until they either tire of punishment or the target somehow manages to produce a result that matches the expectations.

You know what would be handy? If you were to codify these reasons and purposes unambiguously in a single, agreed upon, authoritative place (say, a law book or something), so you don't need to divine meaning from ancient texts and have the same silly discussion every time the subject comes up.

Take a look at the legal system of just about any non-English speaking country, to see how this could possibly work.

(Admittedly, it makes for great film scripts and courtroom scenes, which are valuable and important export-products of the US, but having clearly defined and agreed upon laws and rules is kind of important too)

True. It's also to prevent someone from ever being put in the position of having to choose between lying and self incriminating.

But again, that doesn't apply in this case.

or the target somehow manages to produce a result that matches the expectations.

This is an impossible end state. You might have a point if the password was a one time pad or something like that but that's not the case for the case in question.