> But it then talks about how exp is implemented in hardware, in a way that forces it to be a non-SIMD calculation.

The fact that exp is implemented in hardware is not the argument. The argument is that exp is a library function, compiled separately, and thus the compiler cannot inline the function and fuse it with an array-wide loop, to detect later on an opportunity to generate the SIMD instructions.

It is true however that exp is implemented in hardware in the X86 world, and to be fair, perhaps a C compiler only needs to represent that function as an intrinsic instead, to give itself a chance to later replace it either with the function call or some SIMD instruction; but I guess that the standard doesn't provide that possibility?