It sounds like it has been built that way - the data cannot be decrypted without the correct passphrase - but the user secured the phone with a 4-digit PIN, giving a mere 10,000 possible combinations - easily brute-forceable.

The iPhone prevents this by locking up (and potentially erasing the phone) after 10 failed attempts, but this a restriction created in iOS. If they provision a new backdoored version of iOS to the phone, that restriction wouldn't apply any more, and they could brute-force away.