When people say we don't understand how LLMs work, it's that we don't understand why they take the decisions they do (beyond some quite rudimentary interpretability stuff), not that we don't understand the underlying architecture. If we understood the models we could monitor them and make them behave how we wanted. The recent events pretty decisively show that we can't.
If you say that we "understand the brain" because we understand how a neuron works, you'd be using "understand" in a way that mislead everyone reading that sentence.
If you say that we "understand the brain" because we understand how a neuron works, you'd be using "understand" in a way that mislead everyone reading that sentence.