I can't answer the why, but I can confirm that it's not a typo, the file key is indeed 128-bit as-implemented. However, the "payload key" is 256-bit, derived f

I can't answer the why, but I can confirm that it's not a typo, the file key is indeed 128-bit as-implemented. However, the "payload key" is 256-bit, derived from the 128-bit file key via HKDF-SHA256. It's the "payload key" that keys the ChaCha20Poly1305 cipher, which encrypts the file payload itself.