You could only do a 'true upgrade' from Windows Vista to Windows 7, so how is this any different? I don't think you could upgrade from Windows ME to XP either.
Vista is how old now? It came out in 2006. How many years old will OS X 10.8 allow upgrades from? Snow leopard from 2009.
They aren't saying XP or Vista don't meet the requirements for an upgrade edition, just that you can't do an in place upgrade. Of course you can't, the file structure isn't the same.
This is even better, it means once again you will be able to use the upgrade pricing for clean installs. Good deal!