Now, this is interesting. But I partially agree with the initial posts. Here are some thoughts: 

 1) If the current consensus of linguistic theory is correct, there is no such thing as "complex language" and in fact the human language faculty as a whole has changed next to nothing in the last thousands of years.  
 So the Aboriginal in question would not be "learning language", as his own language differs very little, structurally as well as biologically, from any other human language. His cultural framework is different, of course, but then that's an obvious, almost trivial fact that is only partially connected to the language issue, or at least the LI as you seem to have described it. 

 2) As for the "hard to take seriously" comment, I believe the criterion here is simple: if you are arguing against an established scientific consensus, your claims will naturally appear to be extraordinary, whether you think of them as such or not. And extraordinary claims demand extraordinary evidence.  
 If the evidence is not considered overwhelming in the eyes of the mainstream, it's only natural that many scientists partaking of the consensus will dismiss your theory out of hand. NB I'm not defending knee-jerk reactions, but the fact is they are widespread and perhaps inevitable against all sorts of non-mainstream theories, whether they are plausible or not (and I'm sure that if Bicameralism emerged as the new consensus, this would happen the other way, anyway.)