> Do any forms of thought—our knowledge of the world and ability to reason over these knowledge representations—require language (that is, representations and computations that sup-port our ability to generate and interpret meaningfully structured word sequences)?
Emphasis on "word sequences," to the exclusion of, e.g. body language or sign language. They go on to discuss some of the brain structures involved in the production and interpretation of these word sequences:
> Language production and language understanding are sup-ported by an interconnected set of brain areas in the left hemisphere, often referred to as the ‘language network'.
It is these brain areas that form the basis of their testable claims regarding language.
> Anyone can prove anything and its contrary about language if the term is given whatever meaning is needed for premises to match with the conclusion.
This is why "coming to terms" on the definitions of words and what you mean by them should be the first step in any serious discussion if you aim to have any hope in hell of communicating precisely; it is also why you should be skeptical of political actors that insist on redefining the meanings of (especially well-known) terms in order to push an agenda. Confusing a term with its actual referent is exceedingly commonplace in modern day.