Threads are quite fast, it shouldn't be that shocking that they are as fast or faster than async. What would be shocking is if threads had less memory usage.
So for the c10k problem on a machine with 2 GB of RAM, async will win because threads will exhaust the memory of the machine. Give that same machine 200 GB of RAM and threads may end up being faster.
That’s a very simple model of how memory and cpu interact. If you actually end up using hundreds of Gigabytes of memory, there will be implications on cache hit rates, TLB misses, page table sizes and many other things that make me wary about guessing performance in such a case.
There is also the not much discussed issue of having shared resources between all of these threads and the impact of such a threading model on the engineering part if writing such a program. I personally haven’t seen the thread per connection model in a successful large scale server.
So for the c10k problem on a machine with 2 GB of RAM, async will win because threads will exhaust the memory of the machine. Give that same machine 200 GB of RAM and threads may end up being faster.