It looks like this is not an ARM core, but a Fujitsu implementation of the Arm v8-A instruction set and Fujitsu-developed Scaleable Vector Extension. Most likely the latter is doing all the heavy lifting.
>A64FX is the world's first CPU to adopt the Scalable Vector Extension (SVE), an extension of Armv8-A instruction set architecture for supercomputers. Building on over 60 years' worth of Fujitsu-developed microarchitecture, this chip offers peak performance of over 2.7 TFLOPS, demonstrating superior HPC and AI performance.
The text you linked to actually says that the SVE was developed cooperatively by Fujitsu and ARM, without, however, going into details about who did what.
https://www.fujitsu.com/global/about/resources/news/press-re...
>A64FX is the world's first CPU to adopt the Scalable Vector Extension (SVE), an extension of Armv8-A instruction set architecture for supercomputers. Building on over 60 years' worth of Fujitsu-developed microarchitecture, this chip offers peak performance of over 2.7 TFLOPS, demonstrating superior HPC and AI performance.