Why does Apple use One-Pass Diffie-Hellman for file encryption?
If you have a look in the introductory paragraph of Section 6.2.2 of the NIST SP 800-56A document, you will find an explanation of how the one-pass key-agreement protocol is supposed to work. 

If I understand the description you are quoting correctly, some public ECDH key corresponding to a static ECDH key pair is used to encrypt files while the device is locked, because the actual symmetric encryption key is only available when the device is unlocked. Furthermore, each file gets encrypted with a separate per-file key. This is where the one-pass key-agreement comes into play. In particular, one uses it to derive a per-file key, which is in turn used to encrypt the actual file (one has different keys per file since the ephemeral keys are freshly chosen for each file). Then, as soon as the device gets unlocked one can use the static secret key corresponding to the ECDH key pair to obtain the per-file key using the key agreement protocol and then store/encrypt the file as it would have been stored/encrypted if the device would have been unlocked in the first place.