It's hard to know what people ate long ago. Sometimes, when conditions are right bones might stay around but soft bodied things like berries and fruit can only be discovered if their seeds remain. Sometimes pollen will still be there but rarely. It is only recently that pollen, seeds and the bones of small animals were even sought after and studied. 

The other problem with finding these things is the depth at which these objects are found.  I visited a site that my aunt was working on in Arizonia. The deepest strata was fifteen feet down and represented 3,000 years. In order to find seeds and pollen at all requires a great deal of care, a lot of sifting, and endless microscope slides.

It also requires very sterile clinical conditions so that the dating processes are not contaminated by the inclusion of current materials. Even your breath will ruin a dating processes accuracy. Imagine a result that said that ancient man fed on typical modern fast food that was sprayed on the samples by your breath. 

In this case the discovery of grinding tools is used to approximate the origin of the inclusion of grain in our diets in Utah. However it really is the origin of the grinding of grain. Surely this form of preperation came well after the discovery of the food value of grain.

It is a valuable discovery though because the use of grain in an organised way is a good indication of a change from hunter/gatherer to herder/farmer from campsite to settlements.

http://www.physorg.com/news201788876.html