How seriously should I take the maximum pressure rating of my cyclocross rims?
DT Swiss publishes the exact document you are looking for:

Manuals page / RIMS / Tire Pressure/Dimension (PDF)

The document specifies the maximum usable tire pressure based on rim and tire width. More narrow tires allow for higher pressures. For example, the rims in the question (XR 400, inner width 18 mm) are compatible with tires ranging from 23 mm width at 9.0 bar to 60 mm at 3.2 bar.