This article about YARC mentions that the super computer has 128 threads per processor.
Is the same thing concept as hyperthreading, where essentially the cpu has additional registers that allows to act as multiple processors?
Yes, the physical processor will have lots of registers sets representing virtual CPUs ("threads").
What 128 threads lets the the physical processor do, is put a virtual processor to sleep on an external delay (e.g., memory access), and switch to another virtual processor that is not waiting for anything. This means the physical CPU almost always has work to do, so it is highly efficient. The latency of memory access is hidden so it can be highly non-uniform.
To harness such a system, you need an application program which is highly parallel. If your application has only a few parallel elements, such a processor will not have enough threads that are not waiting on memory accesses, and so it will not be efficient. Then all those virtual CPUs are simply wasted resources (worse, they produce extra heat).