I believe they're using "+/-" to mean "approximately" here. I've seen some people use it in that manner before, though it's always struck me as a bit odd since "~" is less ambiguous.
The way a PCB is manufactured starts with a copper foil of some thickness on an FR-4 substrate, and then the thickness is increased by plating ...
I don't think it's as much of the soldermasks fault as the thermal coefficient of expansion between copper an aluminum, and the large PCB and features.
For aluminum its 21 - 24.0 (10-6 m/(m °C))
For copper its 16 - 16.7 (10-6 m/(m °C))
a aluminum feature that is 5cm will expand about 1.2um/C
a copper feature that is 5cm will expand about 8.35e-6m/C
Yes, it can be used for that purpose, and it works well. You need to remember that the pictures are small, so several will need to be taken, with some overlap. Ideally you'd want the X-rays taken with a digital sensor, so there'll be no wait to develop the prints, and it should cost much less.