Yes the Amiga had a form of UMA as did many other systems, the term UMA seems more widely used than "shared memory" its definite not just a marketing term.
I don't believe Apple claimed to invent unified memory only that they are taking maximum advantage of the architecture more so than anyone to this point.
Federighi:
"We not only got the great advantage of just the raw performance of our GPU, but just as important was the fact that with the unified memory architecture, we weren't moving data constantly back and forth and changing formats that slowed it down. And we got a huge increase in performance."
This seems to be talking about the 16mb SLC on die cache that CPU,GPU and other IP cores share:
"Where old-school GPUs would basically operate on the entire frame at once, we operate on tiles that we can move into extremely fast on-chip memory, and then perform a huge sequence of operations with all the different execution units on that tile. It's incredibly bandwidth-efficient in a way that these discrete GPUs are not. And then you just combine that with the massive width of our pipeline to RAM and the other efficiencies of the chip, and it’s a better architecture."