If the enclave chips don't have a unique identifier on them after being installed in an Apple device does not mean that they didn't have a temporary identifier...
Also, are you saying that even the machines manufacturing them do not know what they are doing ("... secure enclave manufacturing process is done is such a way that even the manufacturer does not know what the key is.")? Sounds a lot like a PR campaign... I would be curious to know how this manufacturing process really works.
It sounds like the encryption key is generated when the device is running as a combination of the UID and passcode/password, so it's not quite as simple as being able to decrypt everything straight away if you are the manufacturer.
Even so, if they do have the UID that greatly reduces the security of the encryption - especially if you are using a short passcode.
The idea here is that the secure enclave is a small microprocessor with hardware RNG, encrypted memory, and encrypted storage. The secure enclave generates its own key and it never leaves the chip.
Also, are you saying that even the machines manufacturing them do not know what they are doing ("... secure enclave manufacturing process is done is such a way that even the manufacturer does not know what the key is.")? Sounds a lot like a PR campaign... I would be curious to know how this manufacturing process really works.