I think you're misunderstanding what the article is claiming. It's not claiming, "Language started in the hands, but we don't know why it left the hands." It'

I think you're misunderstanding what the article is claiming. It's not claiming, "Language started in the hands, but we don't know why it left the hands." It's claiming, "We can't be sure that language started in the hands, because we don't have an adequate explanation for why it would have ever left if it started there." Put another way, this article is summarizing the "started in the hands vs. started in the mouth" argument. The article is very much not on the side of "language started in the hands".