The Cinnamon README file describes this effort as:
Cinnamon is a Linux desktop which provides advanced innovative features and a traditional user experience.
The desktop layout is similar to Gnome 2.
The underlying technology is forked from Gnome Shell.
The emphasis is put on making users feel at home and providing them with an easy to use and comfortable desktop experience.
So rather than using the Mate Desktop Environment or another GNOME2 code-base fork where you're solely reliant upon maintaining the older stack of outdated GNOME technologies for the Linux desktop, they're trying to make GNOME's current 3.x code-base with the Shell look and act more like GNOME2. This comes after the Mint developers already started the Mint GNOME Shell Extensions and other changes to address the large number of GNOME users unsatisfied with the 3.x Shell.
The code to the Cinnamon Shell is available on GitHub. Cinnamon is still early on in its development life.