Isolation between AGND and DGND in the 100 MHz or even GHz range isn't but an illusion. It neither works on the PCB level, because the planes are coupled planar structures nor on the chip level, because both are implemented on a common substrate, either directly connected on chip or maximal isolated by a diffusion well.
Separated grounds serve a purpose for power supplies or for signal circuits in the DC to 100 kHz, possibly low MHz range.