The duress pin deletes the encryption key information used to decode the documents and does not damage the documents themselves.
Not sure why the police and news are saying that he destroyed evidence, since the evidence (as it always has existed before the search began) remains on the disk.
Someone else commented about the law being “non-autistic”, this is a perfect example of that. The technicality of the encryption keys vs the files is irrelevant, the intent was to successfully prevent a search. The border agents will not be even remotely impressed or suddenly decide to release you just because you say “well the files are technically still there”.
What they should really do is make the decryption key backupable.
Then you can keep it at home, and you can genuinely say that you don't remember it because it's like 128 digits.
And you're not "destroying evidence" in the jurisdiction in question because the phone is already locked. And you can't unlock it on demand.
Perhaps a defence to this is ensure that a copy of the encryption key exists in a location outside the jurisdiction of the state.