Oliver Sack's excellent book "Seeing Voices" seems to argue something slightly different:

"A human being is not mindless or mentally deficient without language, but he is severely restricted in the range of his ideas."

To defend the original metaphor: without language, a person might be akin to a Markov chain generated from the relatively small 'corpus' of individual experience.

While the 'well socialized' individual can draw on the vast range of human experiences shared in language. They have access to a much larger corpus.

They may have access to a much larger corpus, but isn't what they actually hear and read also limited to the relatively small corpus of individual experience? I don't want to underplay the density of ideas in language, but I think it's a common mistake to underestimate communication outside of language.

Communication outside of (spoken) language is still a form of language

That would be one meaning of the word language. It isn't always used that way.