The theory goes that as the grasslands spread our ancestors were forced out of the forests and into the savannahs and evolution took hold, favoring a more upright posture because we could see further and so on. Being upright wasn’t really an advantage in the forests because the lousy trees blocked line of sight. The two developed at more or less the same time. Makes sense.
The problem with her theory is that she failed to take into account that a bipedal species had already been found in the forestlands. It was Ardipithicus Ramidus. The remains date to 4.4 million years BCE although some paleoanthroplogists speculate that the area was mixed grassland and forest. And just when something makes sense. Science, go figure!