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 at the loss of a child that causes the parent to protect the child, or to be more risk-averse on the child's behalf. Their imagined grief is what causes the differential in gene-survival rates, and so imagined grief is what we should expect to correlate to the future reproductive potential of the child.
Of course this would be a moot point if people were perfect predictors of their future mental states, but we already that to be false.
An afterthought: Wouldn't it be nice if we could have imagined grief, but not actual grief? I guess evolution couldn't figure out how to make that happen.
Wow, I sure put my foot in my mouth there. Remind me to have coffee before posting. :)