What he is talking about is Security Identifier (SID) and is specific to Windows. Each Windows machine must have a unique SID in an AD environment. Cloned machines will have identical SID which is bad.
The solution to this problem is sysprep, a Windows tool that will reset SID and other parameters so that each Windows installation is uniquely identifiable. This is something done inside a machine, of course, and is not something that can be done on HV level. VMware, and Hyper-V as seen here, have integrated support for invoking sysprep (or equivalent) during the machine cloning process. This is achieved by sending a signal to the management agent inside a machine (e.g. VMware Guest Tools).
As far as I know, XenServer/XCP-ng management agent is rudimentary and does not have this functionality. In world of XenServer/XCP-ng, machine has to be syspreped manually before it is cloned.