Response from MatterHackers:

Walls less than or equal to nozzle diameter: Not printed
Walls between 1 and 2 times the nozzle diameter: Overextruded
Walls greater than 2 times the nozzle diameter: Correct width

MatterSlice's handling of thin walls is something we definitely want to improve.
I'm going to try making a model that has 1.2mm wide walls for my 0.4mm nozzle and see how this calibration technique works with that.