Template "Cases - Send Copy of an Activity" from System Workflow Templates is not available for "Send E-mail" action.
Actually, none of System Workflow Messages can't be seen when "Send E-mail" action is being edited: only User-driven messages are available via the required drop-down. Interesting thing is, then this action has been created for the 1st time, "Cases - Send Copy of an Activity" template was somehow selected ( I did not select anything ) but if I try to edit the action and set custom email parameters, there is no "Cases - Send Copy of an Activity" template available.
UPD: It seems it is Emailapi issue though: line 71 of \emailapi\CRM\Emailapi\Form\CivirulesAction\SendToRelatedContact.php contains
'workflow_id' => ['IS NULL' => 1],
API parameter which name in API Explorer is "Deprecated field for message template workflow".