News The DOE Announces El Capitan, World's New Fastest Supercomputer Will Hit 1.5 Sustained ExaFLOPS

The DOE has yet to make a final decision on which processors it will use for the system, which is odd given that it already has performance projections and the design is obviously in the final stages
Not really. They usually spec out what they want, at the beginning of the bidding process. So, it's not weird for them to have performance specs in hand, as long as they're far enough along to have at least one viable bidder who can meet that spec.

The Shasta architecture currently only supports Intel, AMD, and Nvidia CPUs/accelerators, so that seemingly eliminates IBM's POWER or one of the many variants of ARM processors. Cray says it will announce El Capitan's specific CPUs and GPUs at a later date.
Nvidia makes ARM-based SoCs, with their own, fully-custom ARM cores. Nvidia is now even supporting their entire software stack on ARM.