I've listened to tens of hours of NotebookLM, and this doesn't even seem close. If someone had played his voice for me and asked if it sounded like any LLM/bot I was aware of, I would have said no. It would not have even occurred to me that they were thinking of NotebookLM.

As @crazygringo said, David's voice is lower. I think it might have some of the same harmonics, but it has some lower ones too, which make the overall sound come across as lower-pitched. I'm not using technical terminology here, so perhaps someone can jump in with the appropriate terms.