I think the fighting hypothesis advanced in the paper is probably more plausible than hunting. Would be interesting to know if arm length is sexually dimorphic in other primates.
Just found this paper, for instance, where arm length was sexually dimorphic in chimps but not bonobos:
Linky https://onlinelibrary.wiley.com/doi/10.1002/ajpa.23004 (pdf http://kar.kent.ac.uk/56827)Objectives
Bonobos and chimpanzees are known to differ in various morphological traits, a dichotomy that is sometimes used as an analogy for evolutionary splits during human evolution. The aim of our study was to measure the forearm length of immature and adult bonobos and adult chimpanzees to assess the extent of age–related changes of forearm length in bonobos and sex–dimorphism in bonobos and chimpanzees.
Materials and methods
As a proxy of somatic growth we measured forearm length of captive bonobos and chimpanzees ranging in age from 1 to 55 years. Measures were taken from subjects inserting their arms into a transparent Plexiglas® tube, a novel technique facilitating repeated measures of nonanesthetized apes in captivity.
Measures from adult females (>12 years) showed that bonobos exceed chimpanzees in terms of forearm length and that sexual dimorphism in forearm length is pronounced in chimpanzees, but not in bonobos. Forearm length increased significantly with chronological age in bonobos. Validation tests revealed that the device generates useful data on morphometric dimensions.
In most primates, sexual dimorphism in body size is male‐biased and the differences in forearm length in chimpanzees follow this trend. Given that males of the two species did not differ in forearm length, the absence/presence of sexual dimorphism of this trait must be due to differences in somatic growth in females. Our novel method offers an alternative to obtain morphometric measures and facilitates longitudinal studies on somatic growth.
ETA they don't really speculate on evolutionary causes AFAICT, though they do note that in general chimps show sexual dimorphism (as do humans) and bonobos tend not to, so the arm length result wasn't massively surprising. Anyway, this does rather suggest that the explanation we're looking for could be for the common ancestor of all hominins, in which case tool use is probably not much of a factor and I don't know how much we know about hunting.