My guess is that that's not the full list. It's odd enough that this list is in MP3. So while it's surely reliable, I think there's more to come. maybe unlockable cars via DLC (GOD, NO!) or story mode? Or simply, these are the cars that will make it for sure into the game but there's more to come.
My other speculative guess? GTAV will be released next May. Seems like Rockstar's style. I mean, Red Dead Redemption was released 2 years ago in May. MP3 was released this May. GTAV could be officially announced during E3 and scheduled for released for Spring 2013. I'm pretty sure it won't be Xmas, for they wouldn't want to compete against all the Xmas releases (Blops2, Medal of Honor, AssCreed3, etc).
Captcha: Good evening.
And good evening to you too, sir. My, what a polite robot.