The guy, who a year before anyone ever heard the name Titan was already speaking of a Project Titan being developed by Blizzard, and who claims to have contact to someone at Blizzard, says that Blizzard wasn't 100% honest about whether Titan is going to be a new IP. He says it's going to be a spin-off of an existing franchise. He hints that it might be Diablo-related. You can read his original posts in the article.
edit: 2 things are frequently said by several posters.
1. Blizzard said it's an original IP
Yes, they did, but this topic is about how someone with an inside source claims, that this is not entirely true. If there wasn't at least something like this as a basis I wouldn't have made this thread. So please read the linked article.
2. Where have they said it's an MMORPG?