That's obviously not true. You can do anything you want with a sandbox. Open a socket to the OpenAI servers and then pass that off to the sandbox and let the sandboxed process communicate over that socket. Now it can talk to OpenAI's servers but it can't open connections to any other servers or do anything else.

The startup process which sets up the original socket would have to be privileged, of course, but only for the purpose of setting up the initial connection. The running LLM harness process would not have any ability to break out of the sandbox after that.

As for spawning unsandboxed processes, that would require a much more sophisticated system whereby the harness uses an API to request permission from the user to spawn the process. We already have APIs like this for requesting extra permissions from users on Android and iOS, so it's not in-principle impossible either.

In practice I think such requests would be a security nightmare and best avoided, since essentially it would be like a prisoner asking the guard to let him out of jail and the guard just handing the prisoner the keys. That unsandboxed process could do literally anything it has permissions to do as a non-sandboxed user.

You are essentially describing the system that Codex (and, I presume, Claude Code et al.) already implements.