As 7nm continues to mature there is significant potential for AMD to grow sales if they are releasing a 2nd gen 7nm product at the same time as Nvidia is releasing 1st gen. Small market share means greater growth potential.
AMD is also in a better position than ever before because of having CPU, GPU, and next-gen custom SOCs consolidated on a single process, which means they are a much higher volume customer for TSMC than ever before, which means priority treatment, and they are manufacturing the broadest range of designs on that node, which means their internal knowledge of it is the best.
3Q is the earliest that they will see any revenue from these products, which is why the are forecasting 2Q decline. 3Q will likely be partial ramp, but 4Q should see a big pop.