You are absolutely correct. Thank you. It appears I misread the abstract, which is worrisome, but I'm glad to have been corrected. I added a note to the original post retracting that statement. I also tracked down another post where I made the same claim and retracted the claim there. I'll have to read things more carefully*.

I am now not aware of any studies which looked at the long term effects of melatonin supplementation. I would not be surprised at all if oral melatonin lost effectiveness over time, though I recall gwern saying he did not see evidence of a tolerance effect. I also think people claim many positive things about melatonin that don't seem to check out (negative things, too, in my case). In particular, I don't see solid reasons to believe melatonin improves sleep quality. It seems to basically only shift the circadian cycle, which can be useful in circadian rhythm disorders and insomnia. I don't have my papers available right now, but I can pull up a few on melatonin's effect on sleep architecture if you are interested. (I'll probably do this anyway just to make sure I read them correctly after the mistake you highlighted!)

* To be honest, when I was reading studies about sleep a year-ish ago, I was very tired. My motivation was understanding why I might be so tired during the day. I should not be surprised if I was sloppy through this process. I decided I'm a long sleeper, and I've been considerably better since trying to get as much a sleep as possible.