Not sure I would take microsofts word for it somehow. That aside however wga validates copies of windows whether they are activated or not, if a stolen or generated vlk is used or a crack is applied, the copy will still be validated and reported as genuine, unless the key itself has been blacklisted. That will give a much lower number of estimated pirated copies than there really are.
That source doesn't really provide any evidence for activation reducing the piracy figures either. As is well known the vast majority of windows sales are pre-installed through companies like dell, which of course are genuine copies. However many enthusiasts including myself have been reluctant to upgrade to vista due to the numerous issues with it, with the group that pirates windows software the most not installing vista, then of course there are going to be less pirated copies of vista and a lower % too, no thanks to activation though. You also only need to look at the mac and linux sections on this forum to see how many people are switching to those os as well.