This is missing the point. The whole point of a "sandbox" in the sense of defense in depth is precisely because it's expected that first-line security guarantees will be broken. Like, maybe V8 has a bug and you can write Javascript that fools it into running arbitrary binary code. But that's less dangerous, because the V8 engine runs in a process sandbox and doesn't have access to the rest of the browser state.

Which is exactly what happened here, as it turns out. This isn't a breakout (or isn't alleged to be a breakout) to the broader OS, only to the sandbox environment.