I also am very confused by this because per apple's support page it only supports 4K via HDMI at 30Hz but SHOULD support 60Hz via a mini display port 1.2 specification built into thunderbolt 2.
However, I think it does include 60Hz support (although not mentioned on apple's website). My evidence of this is that on the ifixit teardown they found a an Intel DSL 5520 Thunderbolt 2 controller which according to Intel's and Wikipedia's website is falcon ridge which means it should support Display port 1.2 natively. Plus, on apple's thunderbolt page they specifically mention connecting a 4K display to a macbook pro through the thunderbolt port (not which is suggested by the support page listed above):
"Now with Thunderbolt 2 built into the new Mac Pro and MacBook Pro with Retina display, you can connect the latest 4K desktop displays and get double the bandwidth for your peripherals. And the two generations of Thunderbolt technology are compatible with each other."
Also, the Apple mini displayport support page has not been updated since 2012 but I believe it is just showing old information
The BIG piece of evidence against the new macbook pro's supporting 4K through the thunderbolt port is that on apple's tech specs page they specifically mention 4K under the HDMI section but make no mention of it under the thunderbolt 2 section.