I set the default unit of measurement to mm. The only way I've ever learned to use mu is as a symbol for averages. It seems out of place here to me. What does it mean? I expected it to be the default unit of measurement instead.
µm is a unit of measure just like meter, millimeter and kilometer.
It's not out of place as it's a measurement
You've set the default measurement to mm, so you would expect mm. It's quite possible you selected the wrong measurement by accident.
you showed an image of the solidify modifier showing um, so now I've tried to recreate what you showed, and get the correct measurements in mm, then I selected the um and got the measurements in um.
So the error you are having is quite possible that you have selected micrometers instead of millimeters