There are 3 ways this could happen. The first and most obvious is that Apple is on Intel’s inside track and could debut Thunderbolt 2 devices earlier than the rest of the world. But it turns out there are two other possibilities:

Both HDMI 1.4 and DisplayPort 1.2 have been around for quite some time and both support 3840×2160@30Hz.

In the case of HDMI, Apple could continue to include a native HDMI. This would have to be right off of the video card and not off of the Thunderbolt controller.

In the case of DisplayPort, all the pins are available in a miniDisplayPort/Thunderbolt port, but in the same way mDP displays have to end a Thunderbolt chain, a DP 1.2 4K display would have to be the only device, plugged straight into TB port, with no Data signal, just video.

DisplayPort 1.2 on Wikipedia
HDMI 1.4 on Wikipedia

This begs a bigger question, however. Is 30fps enough? 24 is the bare minimum for movies to look right, games need a little more depending on the type of action, but your OS X UI is usually running at 60.

You might also be interested in what is/isn’t retina when we’re talking about desktop monitors vs laptops.
http://isthisretina.com