OK, so it was just a mistake on CDP/GoG's side, as I just received another e-mail:
Hello again fans of The Wicher,
Sorry for inundating your mailbox again, but probably because of some evil gnomes there was a mistake in the last newsletter which could create some confusion. The pre-order offer for The Witcher 2 at GOG.com indeed includes 5 classic RPGs to choose from as a free gift that comes if you buy the new adventures of the brave Geralt. The games are, exactly as it was stated on the GOG.com website from the beginning: Beyond Divinity, Divine Divinity, Gothic 2 Gold, Realms of Arkania 1+2 or Realms of Arkania 3. This is one of the sweetest deals out there!
We're sorry for any confusion the last newsletter might have created. Please keep an eye on The Witcher 2 and GOG.com!
Michał Kiciński, co-founder of CD Projekt
Case closed ;-)