However, Exchange Online has clearly been added as an additional forest:
I had not executed the HCW for a while but know for a fact that it functioned correctly at one time (perhaps several months ago).
An online search led me to this Microsoft article:
You must add your Exchange Online tenant as an additional forest message
We learn that:
I then re-add Office 365 as an Exchange forest:
I enter my credentials when prompted:
And Office 365 has been re-established as an Exchange forest in the EMC:
We then enter our credentials for both on-premises Exchange and the Office 365 tenant:
Connections and credentials are validated (successfully in this case):
We select our Hub Transport servers (if they are not already selected automatically):
We indicate the organization FQDN:
And we are "ready to update" (Click "Next" or "Finish" as appropriate):
The HCW is working...
And the process completes: