In the link that I cited, the author says:
“the molecules in hot water, which is less dense than cold water, continue to move and in doing so cool quicker than cold water, which remains static.”
If the water is less dense, can we infer that something happened at the molecular level?
No, nothing “happened”. Hot water is less dense because it is hot. That’s the nature of hot fluids. When it cools it becomes more dense. Also, you are quoting the writer of the article, not the guy who did the experiments. If you want to quote something, it would be better to quote the actual paper, rather than your article which is merely a summery of it.
…summary…