@Wilhelm_Stenvall
The premises and beliefs surrounding monsters and mythology are usually one or more cultures' specific interpretations of natural phenomena they didn't understand at the time. Why are you freaking out about this one?
Lycanthropy, Vampirism, and the Undead were diseases and/or decomposition phenomena, and dragons are dinosaur fossils. Fossils and diseases existed outside of the cultures and time periods that spawned supernatural explanations for them. This is well known.
So reasonably, if someone described how corpses used to be prepared to prevent vampirism, one wouldn't interpret that as implying all cultures prepare corpses in similar ways due to Western influence.
The chapter even started by mentioning this was Chinese culture surrounding the topic, and it NEVER claimed some piece of Asian folklore is the only reason people sleep on their sides.
Historically western culture also had some BS beliefs about sleeping posture analogous to this. It was just based more unfounded medicinal/religous beliefs, and associations with death rather than nightmares.
Not to mention some of those older European beliefs favored a half-sitting posture which isn't nearly as common today. Hence it's less relevant to this particular chapter and setting.