A 6670 is overkill for those tasks. Think about the size and TDP (and therefore heat output) of a 6670- why would MS go for that when it could have used a far cheaper, cooler and less power hungry chip for the job?
In addition, we all thought the 6670 was the full GPU for gaming right, and now it might be only the ancillary graphics? What about the game demos- were they run on 6670 equivalent or 7850 equivalent? I guess we don't know yet!
Potentially overkill - I'm a bit out of touch in regard to how well the lower end AMD cores handle things like high def video post-processing and/or what requirements some of the other OS features have. The TDP, etc. of the desktop version and whatever version is implemented in the SoC though could be quite different.