The line output ground terminal has indeed a 22 ohms resistor to real ground.
The reason is to avoid ground loop noises.
Without any countermeasures in the design, here is a common scenario that would cause significant ground loop noises:
- Axoloti Core is connected to a computer's USB port
- Axoloti line out is connected to an unbalanced line input of a device that is also connected (perhaps indirectly) to another USB port of the same PC.
The current consumption of Axoloti Core fluctuates with processor activity, sdcard activity, and perhaps a midi controller with some blinking leds.
Imagine that the USB cable to Axoloti has a resistance of 0.2 Ohms, and the amplitude of current consumption fluctuation is 100mA, we'd find a 20mV signal between PC ground and Axoloti Core ground. That signal will likely be annoying in the audible spectrum.
Unless it is compensated for, and that is what the line output circuit does. It uses the TPA6132A2 to convert the audio DAC balanced (and biased) output to an unbalanced output that is not ground referenced but "soft"-ground referenced. And it does this DC-coupled as a bonus.
The 22Ohms resistance between line out ground and Axoloti ground prevents supply current (and fluctuations therein) to be carried over the audio ground wire, as there will be a different path with a lower resistance for supply ground current.
This application note from Texas Instruments probably explains it better.