I'm not convinced the result is as important here as the methods. Separating language from complex cognition when evaluating individuals is difficult. But many of the people I've met in neuroscience that study language and cognitive processes do not hold the opinion that one is absolutely reliant on the other in all cases. It may have been a strong argument a while ago, but everytime I've seen a presentation on this relationship it's been to emphasize the influence culture and language inevitably have on how we think about things. I'm sure some people believe that one cannot have complex thoughts without language, but most people in speech neuro I've met in language processing research find the idea ridiculous enough they wouldn't bother spending a few years on that kind of project just to disapprove a theory.
On the other hand, further understanding how to engage complex cognitive processes in nonverbal individuals is extremely useful and difficult to accomplish.