The phenomenon described in the article does not require any non-naturalistic explanations. This is pretty much the role that people ascribe to DMT all the time, which is supposedly secreted during our final moments.

I was in the presence of my mother during her final night and it was the most harrowing and traumatic experience I've ever had.

It was obvious that she was in pain towards the end, but interestingly over the 1-2 hour period after she died, when when she was surrounded by friends and family, her face gradually changed into the most angelic and peaceful smile. Even the guy from the cemetery commented on it, as most of the faces he's seen are contorted into painful visages. A naturalistic explanation would be that it was because of her positive personality and constant use of her smiling muscles led to those muscles being the strongest or first to contract during rigor mortis. I kind of like the idea that your death face is a reflection of who you were in life.