PCB plane cuts for audio paths in mixed-signal device
The mic area is probably isolated that way because it can have a very high output/input impedance, making it extremely sensitive to voltage noise. It looks like it has to traverse the whole circuit board (from edge to middle) without any buffering... this would make it extremely succeptible to both digital signals and audio crosstalk.

Additionally the headphone out can feed a relatively low impedance, so this could cause voltage noise across the ground plane. (ground bounce, ground loop)

This isolation scheme keeps this at bay by keeping both unwanted voltages and unwanted currents out of sensitive areas. As you point out, it may not be necessary depending on what you're doing and how you're doing it, but the dev board designer felt it was the best way to go for the demo.