The most obvious inelegance of this study, as described, is that it was conducted by asking human adults to imagine parental grief, rather than asking real parents with children of particular ages.
But why would evolution seek to fine tune actual grief? Evolution requires a difference in rates of reproduction between differing phenotypes. I don't imagine there would be any difference in reproductive rates between those parents who happened to experience no grief at the loss of a child and those parents who were grief-stricken.
It's in fact the imagined grief...
Wow, I sure put my foot in my mouth there. Remind me to have coffee before posting. :)