The Hard Processing System is a quad-core Arm Cortex-A53, which allows users to easily migrate existing SoC designs from Intel Stratix 10 SoCs to Intel Agilex SoC. Both ARM and Intel has their own chip architecture. That will depends on the technology advancements, currently the Cortex-A53 processor is greatly an efficiency processor.
More info on Agilex FPGA:
Well, based on my knowledge ARM processor is popular in low-power consumption device such as smartphones, as compared to Intel's processor which focused more on computing power (PC). Thus, there are advantages when using ARM processors in Intel FPGAs not just for it's low power consumption.