In spectroscopy there is such thing as stray light. Even though how narrow and specific the wavelength is coming from the HCl, the instrument is not perfect enough and there will be other wavelengths of light that will pass thru the detector. Hence, why the monochromator is still needed.
You must bear in mind that the atomization source (flame or graphite furnance) or any other specie present also emit light. The main goal of the monocramator is to eliminate all the unwanted wavelenghts thus making feasible the detection of the wavelenght of interest without interferences.
Hollow cathode lamps in AAS emit multiple lines characteristic of the elemtn, superimposed with the spectrum of the gas. That is why you see a red beam from a hollow cathode lamp. Monochromator is still needed to select a given wavelength.
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