Isn't the Win7 install disc is different than an upgrade disc? The complete install disc has exactly two options (at least, mine does) - upgrade or custom install. If you could perform a custom installation with an upgrade disc, how is it any different from a regular install disc?
Edit: Anyway, just remember that a custom install is equivalent to a clean install, which means that you are not really "upgrading" from XP to 7. I'm merely confused that the FAQ claims that the upgrade disc can do that. Note that a custom install does not require a previous version of Windows to be installed. It does not mention this anywhere.
Not interested in arguing this further - why don't you just try it out and tell us what you find?
v By the way, reading your "XP, vista, even NT" part directly contradicts the "Note" under the first section in your screenshot above.