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I think TxnID is enough for the requester to find out which request the COMP is corresponding to. Is DBID redundant here?
You're correct that the Requester can use the TxnID to establish which request the Comp message relates to. However, you can think about DBID as being the TxnID that the Home applies to the transaction for its own identification purposes. This is then required in order to establish, at Home, to which transaction the potential CompAck message relates.
If the transaction does *not* have a CompAck, i.e. it's not an OWO Write transaction, then it is fair to state that the DBID is then redundant, as there is no resulting message send as a result of seeing Comp.
We are exploring ways to detail this in a future specification update, following analysis of all messages and fields - i.e. not just a single point fix for this field in a non-OWO write flow.