There is an autistic UK adult (I forget his name) that was able to learn to speak Icelandic in one day.
Now I know Icelandic and English related languages but still it's pretty impressive.
I think the problem is underestimating the potential of the neocortex of certain individuals.
For every square centimetre of the neocortex a baby will make 30,000 new synaptic connections per second for 4 years.
This should not surprise us; in that period babies turn themselves into walking and talking individuals with personalities.

PP