The PD300 series of photodiode-based sensors are calibrated with a full spectral curve using a scanning monochromator (plus a few laser "anchor points").
The wavelength ("Laser") setting tells the meter what wavelength is being used and hence what calibration factor to apply when a measurement is underway. It does not, however, physically limit the possibility of other wavelengths from entering. All light (within the sensor's specified range of course) entering the detector will be measured; the meter will apply the calibration factor meant for the selected wavelength, "thinking" that only that wavelength is present.
In other words, these sensors assume a monochromatic light source. Their relative spectral response is not flat and they are therefore not suited for broadband beams.
So, if you want to check one wavelength from a broadband source, you will need to use a wavelength filter that only passes that wavelength. Then you should set your meter to the appropriate wavelength to account for the detector's relative sensitivity.