The only way this is true secure is that this OS prevents ANYTHING from going out. It cannot make any arbitrary outgoing internet connection. It only has a downstream copy of your data that you are free to manipulate or search from within the system. The only output it has is the desktop GUI.

Other than that, it is not secure for an LLM to use because I could be given a document that talks about how to break the system and it would follow the instruction in there dutifully.