How so? This method (monitoring connections through network proxy) is perfectly viable for detecting outbound connections from a browser. If a browser makes an unwanted connection to the browser vendor and/or third party site, that is unwanted telemetry. Regardless of what the purpose of that request was, user's IP address has been transmitted (hence telemetry from user standpoint) and users's privacy was not respected. Client source code is irrelevant from the standpoint of privacy, as such requests are processed server side and to my knowledge no browser with telemetry has open sourced their server side data processing code.