>>... Gannon and her colleagues were stunned by the results of their experiments...
Speaking as someone who's put time in to learn acoustics, I'm just as surprised.
That said, there's decent evidence that Lucy was incapable of speach as we know it. There's no sign of a hyoid bone and the base of her skull indicates she had an ape-like vocal tract, a high larynx and a short pharynx which limits the range of sounds she was able to produce. But there's also evidence that great apes had some kind of hyoid (behind a paywall, sorry) so the question becomes how long might it have taken for evolution to produce a morphology that enables what we'd recognize as speech (hyoid and larnyx/pharynx). IMO, this current work seems to show that there would have been selection pressure (what drives evolution) for these capabilities to be further developed - so your 'grandmothers' hypothesis fits.
I'd also point out that orangutans had the same kind of evolutionary 'deep time' to end up with something parallel. And I wonder if it staying in the forest factored into their not now having it.
We live in a fascinating world...