In the specific case of POWER 8 and 9 the cores are seriously overprevisioned with execution resources and you really need at least 2 threads running in order to make full use of them.
Could be. But the point was more that there's a very thin regime between "waiting for DRAM latency too often" (where more threads can help) and "bound by DRAM bandwidth" (where they won't). The DRAM isn't nearly as parallel as the cores are and saturates really fast.
In that case it was even more serious. Like the PPUs of the Cell processors, each core runs one instruction every other cycle. If you have only one thread, you effectively have half the throughput.