Math is just a subset of language.

When people say "Bitcoin is backed by mathematics", they don't mean the language.

You are talking about "representation", but I think the topic is "form". Mathematics as a language is just a projection of forms that exist regardless. These forms, or let's say mathematical facts, are what we are talking about. Cryptography would work the same it does now if all humanity disappeared, and even if we have never existed, it would function the same.

However though, Bitcoin's security depends on scarcity of computing power and some assumptions of human behavior, which is why the claim "Bitcoin is backed by mathematics" is ultimately wrong anyway.

While there are physical forms within the human brain that map pretty well to mathematics, there is no direct evidence outside of our minds of the existence of mathematical forms. We have been pretty successful at predicting the behavior of nature with mathematics, but old theories since disproven successfully predicted a subset of natural behavior as well.

Can you show me how nature uses cryptography outside of humanity? If it isn't used by nature apart from humans, how can if function at all without humans?

I agree there is more to bitcoin backing than math. Physics and psychology are certainly involved.