Insiders have reported on this previously. Basically the "new" rendering engine is a fork of Trident which removes backwards compatibility and legacy code. They might call it something else, but the new engine is basically Trident, refactored, and without the legacy cruft.

Source: http://www.neowin.net/news/internet-explorer-12-big-changes-...

Validated by a few other insiders, including Paul Thurrott, Tom Warren (The Verge), and Mary Jo Foley (ZDNet).