You need to create a new Runbook using Runtime 5.1.
Funny you should mention this, I just so happened to be researching this yesterday, only to come into work this morning to find a brand new search result! I am posting this less than 10 minutes after figuring out the fix.
Anyways, I did the exact same thing you did, except I troubleshot by using "Get-Module -ListAvailable" in a runtime 5.1 and a 7.2 runbook. The ExchangeOnlineManagement module did not show in 7.2 but did in 5.1.
After confirming the module was showing in the 5.1 runbook, I then tested the actual "Connect-ExchangeOnline" cmd and was able to see that it connected.