Note that it is a 4-core CPU which employs simultaneous multithreading[1] (hyper-threading in this case) to achieve a higher throughput while executing independent instruction streams. This technique can provide multiple hardware threads to the OS to schedule its processes on (typically 2 per physical core), and the CPU takes care of executing them as concurrently as possible on a single core (typically by employing a few hardware enhancements within the core so that it can work with multiple instruction streams in a since clock cycle).
At first, the throughput up to 4 cores increases linearly because 4 OS threads can utilize 4 hardware threads independently, making the greatest possible use of available hardware resources. Beyond 4 OS threads, simultaneous multithreading comes into play and up to 8 OS threads get scheduled on 8 "simultaneously multithreaded" hardware threads, which offers increased throughput, but not as drastic (see how the author uses the word "slightly"). Beyond 8 OS threads, the throughput will not increase as you are out of hardware threads which actually execute your instruction streams independently. The OS can spawn an arbitrary number of threads beyond 8, but they will take turns executing on your 8 available hardware threads - no gain in net throughput. You get limited by the hardware.
[1] https://en.wikipedia.org/wiki/Simultaneous_multithreading