A sandbox is meant to be a controlled environment where you can execute code safely. Browsers can access your email, banking, commerce and the keys to your digital life.
Browsers are closer to operating systems rather than sandboxes, so giving access of any kind to an agent seems dangerous. In the post I can see it's talking about the file access API, perhaps a better phrasing is, the browser has a sandbox?
just make a separate user profile without your email , banking, and commerce, if that's what you don't want it to have access to.
That is like saying the kernel/sandbox hypervisor can access those things. The point is that the sandboxed code cannot. In browsers, code from one origin cannot access those things from another origin unless explicitly enabled with CORS.