Seems like "both libraries make copies of the data, which means you’re using even more RAM." is a assumption (that copy data leads to more RAM usage) and I'm guessing the libraries (haven't used them myself, nor python, so not sure how feasible this is) can be implemented like a persistent data structure (https://en.wikipedia.org/wiki/Persistent_data_structure) and therefore get both immutability (which leads to less bugs, no mutability) and not that high RAM usage. Clojure is a famous example of something that implements persistent data structures.
If the data structure has a reference count of one, you can safely mutate the data structure instead. Many persistent immutable data structure libraries use this as an additional optimization.
