They didn't say LLMs are fundamentally nondeterministic. They said there's no way to deterministically guarantee anything about the output.

Consider parameterized SQL. Absent a bad bug in the implementation, you can guarantee that certain forms of parameterized SQL query cannot produce output that will perform a destructive operation on the database, no matter what the input is. That is, you can look at a bit of code and be confident that there's no Little Bobby Tables problem with it.

You can't do that with an LLM. You can take measures to make it less likely to produce that sort of unwanted output, but you can't guarantee it. Determinism in input->output mapping is an unrelated concept.