The authors claim that modern 15th century people already knew the "pre-historic" narrative about rocks being axes. They are shoe-horning this theory into the painting by claiming the painter and 15th century people already thought these rocks were ancient handaxes, and thats why it features so importantly in the mysterious painting. That seems a rather hoaxy claim to me: The archeological narrative must be correct if people already knew it to be true!
The authors state in their abstract this is their goal, to push back the evidence of such a "social history", to a time before archeologists made up the narrative about hand-axes. The authors are literally attempting to incept the archeological claim to a prior age of modern history. Is that what you mean, because that is definitely and literally the motive of this research.
Edit, also the rock in the painting is not a "biface". That part of the painting has obvious depth and perspective (shadows, walls, angles). The rock in the painting is ROUND.