I think you are conflating the ISA and the microarchitecture. ARMv8 is an ISA, which is implemented by ARM Cortex-A5x series microarchitecture and several others.
Most importantly the X-gene (which these servers are made of) is afaik not based on ARM IP cores, and are indeed built "from the ground up" for datacenter purposes.
I said microarchitecture because ARMv8 is incremental over ARMv7 and not a from the ground up design, but you're right its an architecture. I think the terms are used interchangeably. Implementing 64 bit is a big leap but since its also backwards compatible, it carries improved/specialized ARMv7 components and instruction set (and X-gene and other custom cores per manufacture/microarchiture).
Most importantly the X-gene (which these servers are made of) is afaik not based on ARM IP cores, and are indeed built "from the ground up" for datacenter purposes.