There are real questions about the veracity of some of these Helen Keller quotes, and reason to believe that a lot of it was really coming from Anne Sullivan.
There are really three main observations that give pause. First, after she started working with Sullivan, Keller apparently developed a writing style and nuanced opinions reminiscent of a college-educated person virtually overnight. That’s pretty surprising for any child, let alone one who had only recently acquired language. Second, all of that disappeared overnight when Sullivan died - Keller’s writing style changed drastically, becoming much simpler, and all that eloquent insight also disappeared along with Sullivan. Finally, we’ve never since seen another Helen Keller. On the contrary, later observation of deaf-blind people who don’t acquire language until later in life is part of what inspired a variant of the critical period hypothesis that posits that people who don’t learn their first language (doesn’t matter whether it’s spoken or signed) in early childhood are never able to acquire any language to a particularly high level of proficiency.
That said, even if that Keller quote is now considered dubious, it may not be entirely off base. Supposedly people who don’t acquire language in early childhood also tend to show less capacity for abstract reasoning tasks. Which is still a far cry from the “no-consciousness” that Keller-maybe-Sullivan describes, but does still suggest that language and reasoning are mutually supportive.