Noise is cause by eddies, and there will always be eddies where air streams at different speeds mix, like the outside air and the high speed gasses from the engine.
Anything that will spread out the area where the mixing occurs is likely to reduce the perceived noise, as the mixing is more progressive.
It does work, it has to. Unlike spoilers on road cars (as opposed to race cars) which are there only for the looks (and frequently do not even have the right shape anyway), anything that goes on an aircraft has to be justified and be validated by some testing, as it could have an adverse effect, adding weight.
Does it work that well? That is another matter entirely. Usually, something like that does not have to have a huge effect to be seen as being a step in the right direction. Certification standards and operational restrictions have firm values that are not open to negotiation. If it states the maximum noise is 120 dB (say) and the plane makes 121, that is too bad, but it is a reject. Get an equipment that drops the noise to 119 dB and you're OK.