Wow interesting. I was about to ask how they're supporting 4k60 DP on the models limited to USB 2.0 speeds and how that all works. I decided to look it up though and found this comment;

"The USB Type-C connector allows for up to four high-speed differential pairs, which can be allocated to different protocols. For example, you could implement 5Gbps or 10Gbps SuperSpeed USB using two of the pairs and use the other two for dual-lane DisplayPort. Alternatively, you could have all four pairs used for quad-lane DisplayPort to drive high-resolution or high refresh rate displays.

Notably, there are dedicated USB 2.0 data lines on the USB Type-C connector that are always available, no matter the configuration of the high-speed differential pairs.

So in this case Apple just had to connect their existing USB 2.0 interface to those dedicated pins and run the new DisplayPort interface to the high-speed differential pairs. For the 15 Pro models, it may actually more complicated since they might need to add multiplexers to switch the differential pair lines between the SuperSpeed interface and the DisplayPort interface as appropriate."

(Source is Needleroozer on MacRumors.com, thanks random Internet stranger)