I think it's because "async" has been overloaded. The post isn't about what I thought it would be upon seeing the title.
I was thinking this would be about using multiprocessing to fire off two or more background tasks, then handle the results together once they all completed. If the background tasks had a large enough duration, then yeah, doing them in parallel would overcome the overhead of creating the processes and the overall time would be reduced (it would be "faster"). I thought this post would be a "measure everything!" one, after they realized for their workload they didn't overcome that overhead and async wasn't faster.
Upon what the post was about, my response was more like "...duh".
Async and parallel always use more CPU cycles than sequential. There is no question. He real questions are: do you have cycles to burn, will doing so brings the wall clock time down, and is it worth the complexity of doing so?