It seems simply to be a high-quality electrical tape, but just to be sure, could somebody take a micrometer to it? I suspect ...
Notice the "X" designation on the SoC, which means that it's an experimental device. TI normally sells "X" devices to customers before they've worked out all the bugs in the chip. These parts are not necessarily up to spec, which is likely why Amazon has had to underclock the processor in the Fire HD. Amazon probably thought they could save some money by using these parts, which likely come from excess stock.
Notice that the Lattice FPGA used in previous MBP models to switch the LVDS display signal between the two GPUs ("GMUX") has been replaced with a TI HD3SS212 DisplayPort switch.
Sorry to ruin the magic, but that "EMI shield" is actually the Wi-Fi antenna. The phone antenna is in the speaker enclosure removed in Step 12. The steel frame acts as a ground plane, not actually as a radiating antenna.