They are misleading you, or you misunderstood.  400-750 nm is the
wavelength range of light in the visible region.  UV light is between
200-420 nm.  Normal glass (not pure quartz glass) cuts off UV below
about 325 nm.  mW/lumen is an energy measurement.  Thus, you are getting
a measurement of the UV energy (milliwatts) per lumen of light.

Converting from other units to mW/l is easy in some case and complicated
in others, depending if the surface area of the sensor is known.   A
Lumen is a unit of light emitted from a point source at one candelpower;
1 lumen = 0.001496 watts.  1 lumen/square meter = 1 Lux; see, you need
the area of the sensor to convert to Lux, which are the units you
probably know.  Generally if you compare one lamps output (in any units)
to another lamps output (in those same units) to use the measurements
effectively.

Timothy Vitale
Preservation Associates
831-684-2731