Language vs. Abstract Thought

This article is really interesting, but I got tripped up by this:

Human mind needs human cognition and human cognition relies on human speech. We cannot envisage humanness without the ability to think abstractly, but abstract thought requires language.

I can't find anything in his references that talks about this language requirement for abstract thought. Is this common knowledge that I somehow missed out on? I've certainly run across intelligent people who mistakenly believed that language is required for all thought. It sounds right at first, but everything I've learned about language and cognition in the past two years makes me think otherwise....