The licensing terms you linked to are for Windows Server VMs. When the licensing documents refer to an "OSE," they are referring to a Windows Server Operating System Environment. They are not referring to other OSs you may be running on the same host (remember, the host can by any hypervisor). Non-Windows VMs are not relevant to Windows licensing, even if you are using HyperV to host them.
The only difference between using HyperV and other hypervisors is: IF you are installing the HyperV role on a Windows Server, and only using it to manage HyperV, you don't have to count that bare-metal Windows Server against your license count, provided you are running Windows Server VMs on that host.
Remember, there is also HyperV Server, which is console-only, and free, but fully functional, and managed just like any other HyperV server. It is a slightly scaled down version of Window Core. You can run as many non-Windows VMs on that as you want, without paying a cent, or pay for Windows VMs according to the above rules.
However, you are mistaken. Windows Server standard allows 2 VMs (plus the host). Datacenter allows unlimited VMs. If someone told you otherwise, that's incorrect: http://www.microsoftvolumelicensing.com/ProductResults.aspx?...