When statements like this are made usually it’s because studies would have been done comparing sleep depth at different temperatures and there would have been a statistically significant increase in sleep depth at 18 degrees Celsius in the tested group compared to groups sleeping at other temperatures.

I have no idea if that was actually done here but that’s usually how these statements are justified.

Of course we are all individuals so it may not apply to everyone but you can say that for everything in medicine. Only thing we can do is look for statistically significant differences between groups in double blind controlled studies after controlling for all other factors.