Authentication: "Prove you are you" (hash functions)
Secure Storage: "Keep this secret but let me get it back later" (encryption)
Identification: "Track who/what this is" (UUIDs/tokens)
Password managers—all password managers—require stored passwords to be encrypted such that they can be decrypted. Otherwise they would have no possibly way to retrieve the stored secret for the sake of submitting it to the verifying party.
Best practice for verifiers is to use a one-way memory-hard password hash.
Keychain is a password manager.
This is what keychain does. You retrieve the passwords later.
So, no. It is not a one-way hash function as you stated.