As pointed out early in the article, I had always read it was the other way around; their brains are advanced enough for rudimentary speech, but they lacked the vocal "hardware" that humans developed over time.
The fact that they can learn to sign and show true understanding of a non-verbal language seems to contradict the article's gist.
It is sometimes argued that we do not know enough about their motivations to make that call, but it is hard to imagine there being no situations in which they would find language useful, and even harder to imagine not being able to devise experimental situations in which a greater use of language would be the only way to achieve goals they manifestly desire. Ultimately, the argument about different motives is one about different mental capabilities, anyway.
Personally, I think it probable that species other than Homo Sapiens made good use of language, but we seem to be the only one around at the moment.
That makes it sound like it might be feasible to "fix" this. Maybe by implanting electrodes to stimulate these specific muscles and have the electrodes controlled by unrelated moto-neurons (or maybe capture the sign language and apply the correct stimulus).
How far are we from this, are there any research team working on this? Is it ethical to do it? Is it ethical not to do it if we have the means?