That is totally invalid information, absolutely untrue. Of course you can clean install on a vm, exactly as told in post #3. Thank you @halasz for the voice of reason , you got some rep for that post!
Just to get screenshots now I booted my Windows 10 Build 10525 Hyper-V vm from the install media, ISO I had created by myself from the ESD file, and did a clean install. Originally this vm was created to test if the activation works when upgrading from activated Windows 7 to Windows 10 Build 10525 bypassing the RTM build 10240. That worked as it should, my Windows 10 Build 10525 vm upgraded from Windows 7 was automatically activated.
OK, now just to show how profoundly wrong your statement is I rebooted this vm from the Build 10525 install media to perform a clean install. Naturally, as is the correct way to do this, I skipped the product key:
Next you should not use the installer disk tools to delete the System reserved and possible OEM partitions but simply launch the installation to your previous system partition overwriting the old installation, in this case the old 10 installation which was upgraded from Seven:When the OOBE phase starts after the last reboot with the product key dialog, you naturally skip the key again, do not enter anything. Result, a clean installed Windows 10 Build 10525 on a Hyper-V virtual machine, activated of course:You can now delete the Windows.old folder if you have no plans to go back to the underlying old, upgraded Windows installation.
Windows 10 can and will be activated on a virtual machine exactly as it is done on a real physical machine. A typical user error preventing activation is to try to use a product key; it is not needed in clean install over a previously installed (clean or upgraded) and activated Windows 10.