Could he teach it to the children? If not it would start to seem like an argument that members of the tribe lacked mental faculties that the rest of humanity display (unless severely disabled) and for me would really call his claims into doubt without a lot of research from multiple sources.

I mean, this is the whole point of linguistic determinism, the concept under discussion here. These folks believe that our ability to think in recursive terms (as is necessary when dealing with numbers) is acquired through language.

I don't really believe it, and most linguists haven't since the early 20th century, but that's the claim he made.