Azure AD Connect and Azure DS

We have a customer looking to migrate to Office 365.

They already have a tenant, created as a proof of concept, with Azure AD Connect installed in the root domain. They would like to create child domains for different divisions and host domain controllers in Azure. Is this technically possible as Azure AD Connect will already be syncing objects and the following article suggests it's either Azure AD Connect or AD extension, but not both?

https://docs.microsoft.com/en-us/azure/architecture/reference-architectures/identity/adds-extend-domain