The sequence number seems to be used for locking a transaction in mempool. However, the coinbase transaction is not part of the mempool and thus the sequence number does not make sense.
Is it just an "extra nonce"? Or is there any constraint on the sequence number in a coinbase transaction?
There are no special requirements for the sequence number in coinbase transactions. It is subject to the normal sequence number rules, but for coinbases, those largely don't matter. While it can be set to anything, miners generally just leave it as the maximum sequence number rather than using it as another nonce.
