TOsEnum and COsDescr do not contain information for Windows Server 2016 yet and report such a system as Windows 10.
The TOsVersionInfoExW.wProductType field is set to 3 (VER_NT_SERVER) and should provide the required indicator. (This was also used to patch TOSVersion with 10.2 Tokyo.)
Additional information about the server type (e.g. Server Core) may be available via the registry: Software\Microsoft\Windows NT\CurrentVersion\InstallationType
It would be nice if you could update this field with the next version.