"It's also not clear how any other tool would have detected the long-lived, persistent outbound connection with relatively low bandwidth"
Perhaps, but this extension could have been stealthier. It was using a plaintext web socket on port 6332. If the extension author had instead gotten a Google analytics account, and exfiltrated data via encrypted https GETS to Google servers, it might have never been spotted. That kind of traffic likely happens 24/7 in a typical corporate environment.