You don't have to have a degree in neuroscience to be aware of the fact that we aren't even close to understanding how human brain works when it comes to high-level process such as sentience.
First off we understand a lot about the human brain, but that doesn't matter because no one is arguing that we understand the totality of the human brain, instead what is being argued is: The human brain is as simple as an LLM and thus LLMs are sentient, can reason, and can know facts.