This original quote:
makes no mention that the "20nm production capacity" is only for ARM SOCs. Large die sizes such as GPUs and FPGAs will take up a lot of that 20nm production capacity and AMD is not mentioned as one of those who procured early 20nm production capacity from TSMC.
About a week or so ago, Xilinix announced that they're Ultrascale 20nm FPGA is available for shipping. That's a big chip, but a good sign that commercially viable parts are available (albeit probably very, very low volume). If I recall, I want to say that the first 28nm FPGA were out late summer of 2011 (about 4-6 months before Tahiti shipped).
So I don't think it is completely out of the realm of possibility that we'll see a 20nm GPU in Q2 of next year.