There is more than just this case to suggest this. There were other Feral children who developed to a similar level to Genie (though, as pointed out by another

There is more than just this case to suggest this. There were other Feral children who developed to a similar level to Genie (though, as pointed out by another commenter, it's impossible to say how much of their lack of development is a result of the abuse itself and has nothing to do with language). There is the Pirahã who don't have numbers as part of their language and are apparently unable to learn them passed a certain age (note: AFAIK both these points are contested). Another part of the idea comes from the observation that our brains stop forming new connections at some point. But I think all of this is touched on in the article. I was in the "language first" camp until I read this. Fantastic stuff.