A device can't be joined to an on-prem domain and Azure AD Joined. Sounds like you are referring to enabling Hybrid Azure AD Join (which is just joining the on-prem domain and registering in Azure AD)
Servers can't enroll in Intune, but depending on your configuration, they may get Hybrid Azure AD Joined which isn't a bad thing. Do you have any specific concerns?
That link refers to enabling Hybrid Azure AD Join (not enrolling into Intune). There is a seperate GPO to enroll devices into Intune, and thats only for Windows 10/11 and not Server. https://learn.microsoft.com/en-us/windows/client-management/mdm/enroll-a-windows-10-device-automatically-using-group-policy