Very interesting paper: this is an original approach to comparative physiology, although the data on stomach pH, as you say, are still scarce, e.g. on apes. The interpretation of low pH in human ancestors for carrion feeding e.g. on the African savannas, however, is extremely unlikely, not only because "Not only vultures, but also canid, felid and hyaenid carnivores were much better preadapted for such a niche. They possessed sharp beaks or long canine teeth and did not need to carry stones for cutting carcasses" (Verhaegen 1985), but also because recent paleo-anthropological publications (see below) show that Pleistocene Homo did not run over open plains as often popularly assumed without scientific evidence (google e.g. econiche Homo), but instead dispersed intercontinentally along African & Eurasian coasts & rivers (coastal dispersal, Munro 2010), beach-combing, diving & wading bipedally for littoral, shallow aquatic & waterside foods, especially shellfish, which are uniquely rich in brain-specific nutrients (Cunnane 2005). Although Pleistocene Homo is known to feed on carrion occasionally such as stranded whale carcasses (Gutierrez cs 2001), the waterside omnivorous diet & shellfish in particular can well explain human low stomach pH (e.g. pied oystercatchers pH 1.3, your Table 1).
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