"The study concludes that unpredictable resources, like the coula nut in the field survey, are seen by chimpanzees as more valuable. When these resources are scarce and access to them is on a “first-come, first-served” basis, they are more prone to switch to bipedal movement, because it allows them to carry more of the resource at once."
I think the explanation provided is an impetus for momentary acts of bipedality but it doesn't explain why evolutionary adaptation along those lines would follow...for that, some survival advantage would have to be provided consistently for using that mode of ambulation...and I believe that comes because moving on two limbs instead of 4 is simply more energy conservative.
If it were more risky to ambulate on 2 limbs, say because it is slower or requires more energy to perform than on 4, then that risk taken over subsequent event trials by individuals puts them at danger of never passing on that ability to progeny and if they do succeed in having progeny damns them to a harder life should their primary care giver fall while using a non conservative gate on a consistent basis.
So the inefficiency of one would cascade to the lower generation effecting a stronger anti-selective force for that individuals line. On the contrary if less risk is entailed by assuming such a gate (mind the possibility that chimps (studied) can ambulate on all 4's and carry (rather drag) objects between available fingers...however this motion is cumbersome and possibly more energy expensive as well.)
With a fully bipedal gate, body load no longer need be born on 4 limbs saving energy in the upper body (which now just serves as a counter balance for the pelvic action) and the intentional loss and gain of equilibrium allows balance loss to be traded for *stride length or speed* thus getting more distance covered for each leg swing (or more swings per distance traversed) while expending less energy to do it...now you've got a way to survive better.
This energy conservation enables the arms to be free to carry away resources but also provides speed to escape from the situation should predators or troop mates pursue.
Anthropologists need to look at the work that roboticists have been doing in the area of efficient gates to see why this makes sense...they have shown clearly in the last 20 years that bipedal locomotion with passive dynamic processes (where you let the natural return to equilibrium of a limb provide energy for the detachment of the other limb to continue the cycle) is more energy conservative than many types of greater than bipedal motions with active motorization (equivalent of muscle actions as are required in a horizontal body plan carried on 4 limbs).
They even have mathematical models of describing the efficiency (specific cost). Many learning models for gait cycles naturally evolve bipedal gates as well when given cycles of evolution (evolutionary algorithms they are called) under specific conditions modeling the real world.
So I posit this gain in energy efficiency *is why you keep doing a bipedal gate after you've discovered it* and it helps you then exploit the fact that you've got free hands to use to carry stuff more efficiently and escape quickly (from predators or troop mates in pursuit of your cache) in the same act (if that's the situation you happen to be in) and you get the best of both worlds by taking that consistently winning risk and *now* you have a survival advantage. If it weren't energy conservative...having the hands advantage alone (minus the bipedal gate) wouldn't help you be more efficient at getting away...so assuming bipedality only would have served as a *reason to take risk* and use the gate to aid in escape while freeing hands for carrying...that just happened to be more efficient to do so...and thus conferring the survival advantage. If it weren't more energy conservative, we'd likely still be on all 4 limbs next to our chimp cousins.
It's also important to note the rapidity with which bipedal locomotion has been discovered in the wild, it turns out that homonoid primates are not the first species to do so, that honor goes far back in time...hundreds of millions of years to the emergence of the first dinosaurs, who are believed again to have an advantage (of several) over their sinodont and other Permian families which included a bipedal gate. I don't think their dominance and singular posession of bipedalism are a coincidence...again energy conservation points the way.