Those words are salon's, not the researchers, and they're a headline at that. I wouldn't assume that "it's nurture!" is the ultimate conclusion of this work. I'd suggest they're saying something more along the lines of, "Children who converse less then their peers are less likely to excel".

There is zero doubting that 'nature vs. nurture' is a complicated question (and a false dichotomy) and I don't think this article is making any claims outside of it's pretty narrow (and pretty interesting) focus.