The chips in the supercomputer are not exactly ARM, they do have arm-8 instruction set but that's just for loading the proprietary vector extension unit.
Pretty much a GPU alike stuff with arm-8 set to be able to run its OS.
I do not get the "not exactly ARM" bit. The chip has a lot of cores that execute standard ARM code with SVE instructions mixed in. You can download the manual for the SVE from ARM's website. Following a quick search I also see that GCC has a flag for generating SVE instructions.