[schobi]

A camera+lens set up to 5 micron/pixel will have a shallow depth of field.

I looked up some numbers: The pixels of the camera are 4.6um, so the likely used a 1:1 makro lens (likely the HC 4/120mm). You will capture a 53x40mm region at once. The working distance for this lens goes down to 40cm for 1:1 magnification (might have been 40-45cm). Aperture 4 (as little diffraction as possible)

If we put that in a calculator, depth of field is only 240um. This is the working range where the object needs to be to be in focus.

I'm surprised the painting is that flat over a single image. Even a high spot on the canvas or an extra dab of paint will be higher. Maybe they took multiple images and focus stacked them?