Because that's the name of the chip in the Mac that Apple developed in house. They had that IP hanging around and put it in the only place they could - the security coprocessor.

See sibling comment that the security coprocessor is a repurposed A10. And then to answer the next question, because the HEVC encoder for iPhones has been optimised, and they can likely just use the same code that already exists for iOS.

I have wondered whether they should be using the GPU in the A10/T2 too, instead of the Intel integrated graphics. Would probably perform rather well!