As above what they are are saying is that using Hybrid AD is fine with a domain name that is not the same as long as you add the UPN in Active Directory Domain Trusts and then you adjust all the users in the domain to match that domain either manually or scripted.
https://docs.microsoft.com/en-us/office365/enterprise/prepare-a-non-routable-domain-for-directory-synchronization
My only concern with this is that you might not need to sync AD but if you are already setup then it is kinda hard to revert.