To be clear, it's not that sleeping while horizontal is better than being awake while horizontal. We all need solid sleep, but being horizontal should otherwise be minimized as it is the ultimate sedentary lifestyle.[0] This causes heart atrophy.
I think you’re incorrectly jumping to correlations/causations.
Your references are due to sedentary, not laying down. You can have a sedentary life standing in place most of the day. Likewise, the problem is being sedentary, not what position your sedentary in.
It’s a reasonable hypothesis, but you’d need to experiment to validate it. It’s easy to imagine how prolonged exposure to bed rest or microgravity may trigger heart muscle remodelling in a way that intermittent bouts of lying down would not.
[0] https://www.betterhealth.vic.gov.au/health/healthyliving/the...
see my other comment for other references: https://news.ycombinator.com/item?id=40549893