how can we know ___ by only examining language
I've accepted that language is knowledge. In the case of LLMs, to model language is the same thing as modeling knowledge.
Webster's: (1) information, understanding, or skill that you get from experience or education
If we look at knowledge separately from experience and education - as just the raw knowing - what we mean exactly by "having knowledge" is that the language therein is of a certain (correct) kind and order.
If I can reliably say all the right words, in the right order, about X, then I know X.