The title is bad, it is not sandbox escape, it is "RCE inside the sandbox", so only RCE when sandbox is disabled.

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.