This is a difficult issue. There are very few documented instances of feral children and it is hard to isolate their language deficiency from their other problems.
What we do have a lot of documentation on is children with various types of intellectual disabilities. My four-year old daughter is autistic and has an IQ of 50. Her language is around the level of a 24 month old (though possibly with a bigger vocabulary and worse grammar). Does she have a deficient language module? That doesn't really seem like a great explanation for anything. Her mental deficiencies are much broader than that. If there were a lot of children with deficient language but otherwise normal development that would lend some support to a language module model. But this isn't really the case. If your language is borked that usually means that other things are borked too.
Another thing about my daughter: She's made me realize how smart humans are. A retarded 4 year old is still really really smart compared to other species. My daughter certainly has far more sophisticated language than this guy did. I bet she could beat a chimp in other cognitive tasks as well.
In my article on trusting expert consensus, I talked about the value of having hard data on the opinions of experts in a given field. The unspoken subtext was that you should be careful of claims of expert consensus that don't have hard data to back them up. I've joked that when a philosopher says there's a philosophical consensus, what he really means is "I talked to a few of my friends about this and they agreed with me."
What's often really happening, though (at least in philosophy) is that the "consensus" really reflects the opinions of a particular academic clique. A sub-group of experts in the field spend a disproportionate amount of time talking to each other, and end up convincing themselves they represent the consensus of the entire profession. A rather conspicuous example of this is what I've called the Plantinga clique on my own blog—theistic philosophers who've convinced themselves that the opinions of Alvin Plantinga represent the consensus of philosophy.
But it isn't just theistic philosophers who do this. When I was in school, it was still possible to hear fans of Quine claim that everyone knew Quine had refuted the analytic synthetic distinction. Post PhilPapers survey, hopefully people have stopped claiming this. And one time, I heard a philosophy blogger berating scientists for being ignorant of the findings in philosophy that all philosophers agree on. I asked him for examples of claims that all philosophers agree on, I responded with examples of philosophers who rejected some of those claims, "Ah," he said, "but they don't count. Let me tell you who's opinions matter..." (I'm paraphrasing, but that was what it amounted to.)
I strongly suspect this happens in other disciplines: supposed "consensuses of experts" are really just the opinions of one clique within a discipline. Thus, I tend to approach claims of consensus in any discipline with skepticism when they're not backed up by hard data. But I don't actually know of verifiable examples of this problem outside of philosophy. Has other people with backgrounds in other disciplines noticed things like this?