IANAL; even in the USA not all speech is protected under the law. See libel laws.
Here it is less about the code as speech, but code as an illegal business and product. It's not an either//or situation. Code is more than just one thing.
Code is never an illegal business. You have to run the code to do an illegal business. In this case running the mixer is apparently illegal (wonder if at all or with just some specific cases), but maintaining and improving the code to run such a mixer certainly not.
Working for an illegal business (tornado cash isn't just some code repository, it's basically a SaaS business backed on a chain), on its product, usually makes you liable. That's why people working for cartels are prosecutable.
edit: I believe the term is "criminal facilitation."
Here it is less about the code as speech, but code as an illegal business and product. It's not an either//or situation. Code is more than just one thing.