I have looked up some papers that cite that one:
In this one:
Recently, possible association between an increase in peripubertal fluoride and later pubertal development in boys, but not girls, exposed to fluoride through fluoridated table salt (200–250 mg kg–1 of F–) was suggested.480
Which would be the opposite of what you would expect if melatonin is reduced.
I think I figured out how to efficiently eliminate fluoride from tap water without installing reverse osmosis system. I use 2 different filters: 1. under the sink filter (dual system) takes out 95% of fluoride (Woder, $238 initial purchase, then $170 replacement every 12 mo) and 2. Epic pitch filter (replacement filter around $60 every 3-4 months) removes 97.98% after that. So if fluoride concentration is 0.7 mg/L then both filters have 99.89% total reduction (final concentration is only 0.00077 mg/L).