I declare I am an inexpert amd stock holder, but hopefully objectively, I agree with others here that its just a plan B for AWS to use as an "arm twister".
They said amd's arm didn't live up to expectations, but nor does this it seems.
It bears noting annapurna cost ~$350-370m back in 2015
I think they just got a better deal this way than getting much the same arm twister from amd. 350M$ isn't huge considering their presumed cpu spend - they just bought their own designer team & arm licence.
Its just another logical step in the very slow continuum of the arm solution finding a problem to solve in exascale.
Instead of using amd's workshop & team for their experiments, they got a bargain on their own setup, team & arm licence. Their own ~test kitchen, just as arm instances seem to be for their clients to experiment on.
Nobody seems to be saying arm just took a big leap vs x86.