In looking over the Slippy Git commit for adding the support to Coreboot, this Chromebook is powered by Intel Haswell. There's been talk of a forthcoming Google "Slippy" Chromebook going back to April/May of this year but not too much news recently. Slippy is likely the Haswell refresh to the Chromebook Pixel that has yet to be formally launched.
The few Haswell-based Chromebooks on the market right now are using the Intel Celeron 2955U processor, which doesn't match the information from the Coreboot code on Slippy of having eight threads. While I don't closely follow Chromebook announcements, it sounds like Slippy is the high-end Haswell Chromebook Pixel codename.
Besides the initial Slippy commit adding the mainboard support, additional Slippy commits landed after the fact ironing out more functionality for the Intel platform. Stay tuned for more updates.