The article cites a BlueGene system with 768,000 cores.
This article clarifies it as a BlueGene/Q system.
Wiki said that each BlueGene/Q CPU is an 18-core PowerPC A2 @ 1.6GHz. That's puts Mira between 42 and 43 thousand physical CPUs.
We don't really know what those A2 CPUs can do for hashing, but we can estimate. The
hardware wiki says that a Power7 from 2010 can get 7.6MH/s with 4 cores at 3.5GHz. Who knows if the A2 is faster than the Power7 (for hashing), but we'll go from those numbers.
So 7.6MH/s with 4 cores at 3.5GHz.
That's ~0.87MH/s per core at 1.6GHz.
Final number for all 768,000 cores: ~667GH/s.