Your steps/mm seem way too low...what is your drive method for X & Y?  Most systems using belts and drive pulleys are set at around 80 Steps/mm...and that assumes your stepper driver is set at 1/16.

I agree, it is very strange, but I don't know anything about his printer and he states "built and been running successfully".

It is also very strange the X and Y step/mm are also very different. may result in distortion of the prints. If his correct step/mm is 80 Steps/mm, then it totally make sense the minimal resolution become 0.4 instead of 0.08 (about 5-7 times difference).

#define DEFAULT_AXIS_STEPS_PER_UNIT { 11.94, 13.93, 500, 129.7 }

Taking my X axis for example, this should translate to 1/11.94 = 0.083mm/step. In my host controller - which is Simplify 3D – I tried finding the smallest X movement I could get the printer to move. I assumed this would be 0.08 mm since that would be one single step of the motor. I moved the X axis to 100mm from the home position and then sent a G1X100.1 F1 (the F value was set as low as possible so I could roughly count the steps the motor moved. ) I found that I had to increase the move to 100.4mm before I got any movement of the motor and then the motor moved about 3 or 4 steps to travel from 100mm to 100.4mm. Further increasing the move distance resulted in no movement until G1 X100.8 F1 and then the motor moved the same distance as before.
I don’t understand why the smallest distance I can resolve to is about 0.4mm. I would have expected 0.08mm. Also why is the movement, when it happens more than 1 step? Is there some setting in marlin that is preventing the resolution from going below 0.4mm?