Yes, That can be a state. IIRC you would have configured ADPW or EPW to not change/update the MSExchMasterAccountSid. So while it works now, any new delegation will have only the Target Sid stamped.
It is step 6 that will unlink the mailbox and set it to a user mailbox
Note: sidhistory should be applied during this Migration session. It is not supported by Exchange for sidhistory to be present while the mailbox is linked.