There are several sites in which the remains of robust australopithecines are found associated with flaked stone tools. Because the robust australopithecines were contemporary with later gracile australopithecines and with early
Homo, the majority opinion has generally been that these were not the toolmakers, and not the robusts.
If chipped stone tool manufacture antedates the appearance of the robust morphotype by a few hundred thousand years, this increases the possibility that they were descended from tool-makers. Isotope evidence suggest as well that (at least some)
Au. robustus in South Africa were not living primarily on tough, fibrous plant foods as had long been believed based on their specialised tooth morphology.
To me at least, this all makes more plausible the idea that robust australopithecines were not the ecological specialists they're generally portrayed as. Their distinct jaw adaptations could be selected not because tough plant foods were their primary food, but because they were their fallback foods in time of stress and scarcity.