The point is that the average developers job in achieving this is not knowing every SIMD primitive for every platform. It is almost always folly to ever touch a platform's SIMD instructions unless you're one of the aforementioned library/compiler developers.
Instead the ordinary developer is usually best served by using equipped libraries and understand how to structure code where it can enable vectorization. Whether it's Eigen, or even the C++ compiler (which, in my experience, is often excellent at vectorizing code), or the Accelerate library on Apple platforms. These things mean that your code can adapt and exploit every possible platform they're deployed to, versus Jimmy SIMD who carefully made his SSE 3.2 instructions and his code was then lost in time.