Apparently not even Microsoft believes this, as even they provide
instructions on how to replace one.
http://support.microsoft.com/kb/824125
This is not in violation of the EULA, and is obviously supported by Microsoft.
Although what you say is correct, the web page you cite does *not*
support your point of view.
The web page deals with replacements of the motherboard in general,
not with replacement of the motherboard on OEM systems, which is what
Patrick's comment is about.
An OEM license is tied permanently to the first computer it's
installed on. It can never legally be moved to another computer, sold,
or given away (except with the original computer).
So the question (which has been asked here many times) is what
constitutes a new and different computer. Patrick's comment
("Technically, your XP license died with the original motherboard") is
based on the premise that changing the motherboard makes it a new
computer. For a long time, it wasn't clear exactly what constituted
the original and new computers, and many people felt that replacing
the motherboard made it a different computer.
However, Microsoft has clarified the situation. See
http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/activationfaq.mspx
or
http://tinyurl.com/384gx5
which states
"If you acquired Windows Vista pre-installed on a computer from a
major manufacturer (sometimes referred to as an Original Equipment
Manufacturer or OEM), Windows Vista will require re-activation if you
replace the motherboard with a motherboard not provided by the OEM."
So clearly, if you can reactivate it, it's legal to use it.
However, leaving aside the question of legality, as I said (and as is
quoted below) most OEM copies of Windows are BIOS-locked to the
original motherboard, and changing it to one not identical to the
original will not work with that copy of Windows.