They are misleading you, or you misunderstood.  The 400-750 nm range is
the "wavelength" range of light in the visible region, only.  UV light
is between 200-420 nm and infrared light is between 750-10000nm.  Normal
glass (not pure quartz glass) cuts off UV below about 325 nm.  mW/lumen
is an energy-per-energy unit.  Thus, you are getting a measurement of
the UV energy (milliwatts) per energy unit of light, lumen (for a point
source, 1.496 mW).

Both visible light and UV cause damage.  UV energy can physically break
chemical bonds.  Visible light in the blue region can do the same to
some compound, but mostly absorption of light wares colorants out.  To
create color light is absorbed.  The absorbed energy changes the
colorant temporarily.  Over time colorants fail a molecule at a time and
decolorized.  UV causes significant damage to very light sensitive
materials, but there is not that much of if it in normal room lighting.
Visible light causes all materials to decolorize and there is generally
a lot of it.  Some materials fade more than others.  Its a complex
topic; that was the simple version.

Converting from other units to mW/l is easy in some case and complicated
in others.  To convert to Lux, the surface area of the sensor measuring
lumens needs to be known.   A Lumen is a unit of light emitted from a
point source; 1 lumen = 0.001496 watts.  1 lumen/square meter (of the
surface being illuminated) = 1 Lux, a very small amount of light and
energy.  One needs to know the area of the sensor to convert to Lux.
Generally, if you compare one lamp's output (in any units) to another
lamp's output (in the same units), one is using the measurements
effectively.

Timothy Vitale
Preservation Associates
831-684-2731