That's...not precisely true. The C++ standard doesn't specify how std::async works, and for a while GCC just ran the operation sequentially, and later both GCC and Clang launched new OS threads by default. https://stackoverflow.com/q/10059239/1858225
std::async is mostly "good enough" for ordinary cases where you only want to fire-and-forget and have no control over the underlying dispatching and scheduling algorithms. If you want something tweaked towards your use-case, and hence actual high-performance executor library, std::async is not gonna cut it. And that's where the "C++ executors" proposal come in.