It's worth noting that one of Apple's largest competitor at TSMC is, in fact, Nvidia. And when you line the benchmarks up, Nvidia is one of the few companies that consistently beats Apple on performance-per-watt even when they aren't on the same TSMC node: https://browser.geekbench.com/opencl-benchmarks
Their ability to design a chip and networking fabric which is fast/efficient at training a narrow set of model architecture is not far fetched by any means.