"ARM has developed a prototype architecture based on the Cortex-A core" isn't quite right. Armv8-A (well, Armv8.2-A in this case) is the architecture, Cortex-A is a family of implementations of that architecture. And the Morello implementation (called Rainier) isn't even directly based on a Cortex-A core, it's based on the Neoverse N1, which is itself then derived from the Cortex A76.