You can calibrate the measuring current every time when you think it will be needed (therefore the holes in the frontpanel, to get everytime access to the trim-pots), so it´s not important if it could measure with the same precision as a year before - just trim the current as precisely you can to 100ma and that´s it.
You´re right, DMMs should deliever this because they´re a "black boxes", the customer shouldn´t (and haven´t) play on it´s factory adjustments.
And again yes, the principle of this unit is simple.
But it works, regardless of it´s cheap price.
Comparing to our yearly calibrated benchtop multimeters from keysight, it doesn´t doing anything worser.
Only a few more than 100 bucks, but it works - Sad but true.
But to repeat it, if I get the chance, I´ll test it under different ambient temperatures.