Melatonin won't improve the quality of your sleep. It might induce sleep if your body doesn't naturally produce melatonin, e.g. due to going to bed at very different times throughout prolonged periods. But, outside of that, melatonin isn't very useful as a sleep quality enhancer.
Similar to CBD, IIRC, one would have to take very large amounts of CBD to produce any statistically significant effect on one's sleep, and CBD gummies are not the most bioavailable CBD there is.
My guess is it's placebo rather than anything else.