I am not getting your quoted text via the link. The OEM Partner Center link quite clearly states that if you have OEM or Retail, you use OEM or Retail licenses and media. If you have VL, you have to use the VL licenses and keys.

From the link you supplied:
The downgrade process
Follow these steps to downgrade to a previous version of Windows or Windows Server software:
1. Obtain genuine Windows media and a corresponding product key for the version of Windows that is eligible for downgrade.
The media should come from a prior legally licensed version from the OEM or Retail channels.
End users who are licensed separately through Microsoft Volume Licensing (VL) may provide their VL media and key to a system builder to facilitate the downgrade on their own systems.

http://www.microsoft.com/OEM/en/licensing/sblicensing/Pages/downgrade_rights.aspx#fbid=7rB7Kx4XZaY

Downgrade media and product keys

End users may use any existing software media or product key to install and activate downgrade or down-edition software*. The downgrade software and product key may be from any channel (Volume Licensing (VL), OEM, or retail) and does not need to match the channel license type that is assigned to the server.

Acquiring media and product keys
•End user provided. Customers who have existing solutions or environments with a previous version of Windows Server may use any existing media and product key to exercise their downgrade rights. The downgrade software and product key can be from any channel (OEM, VL, or Retail).
•Microsoft provided. Prior to September 30, 2012, Microsoft provided downgrade media and product keys directly to end users via a Windows Server fulfillment website. Due to low utilization rates, Microsoft no longer provides this service.
•OEM partner provided. Direct OEM partners with a royalty OEM agreement (HP, Dell, etc.) have the option to distribute downgrade media and product keys directly to end users, per end user request. This is optional, so not all direct OEMs will provide this service.
* A down-edition is a lower edition of Windows Server software. For example, lower editions of Windows Server Datacenter edition would include Windows Server Enterprise, Windows Server Standard, and Windows Server Web edition.

In the end if you want to install Essentials as a VM which you are licensed to do you must download Essentials from any legal source along with an install key. Install Essentials then use DISM (http://technet.microsoft.com/en-us/library/jj247582.aspx) to transition your VM’d Essentials to Server 2012 with Essentials Goodies at which point you end up with a broken server as it won’t update properly and reporting is screwed up.

I am floored Microsoft has put in place a process that is so poorly documented and leaves the end user with a broken install.