The actual requirements are titled Windows 8 hardware certification requirements and are linked below.
http://msdn.microsoft.com/library/windows/hardware/hh748188
I see no mention that the manufacturer cannot make other ARM tablets with Android or whatever. The quote given in the article is misleading in that respect.
What you imply doesn't make sense regardless, if ASUS makes a Win 8 ARM device, they would be forced to lock the Transformer Prime's bootloader? How does that even benefit MS and "lock out" Linux? That seems be actually locking Linux in!