The preference controlling whether telemetry is sent anywhere is "datareporting.healthreport.uploadEnabled". The other telemetry settings are just about what telemetry modules are enabled to collect telemetry data locally (which you still can view with about:telemetry, but will not get send when you disabled "uploadEnabled"). [1]

Disable "datareporting.healthreport.uploadEnabled" and Firefox will not send telemetry data around. But users cannot be expected to mess around in about:config, right? That why there is a checkbox in the Firefox preferences just for this. In the Privacy & Security tab.

Also, when you create a fresh Firefox profile (e.g. when you use it for the first time), it will open their privacy policy page, with a somewhat hidden button that brings you straight to those checkboxes. I don't like that it's a bit hidden now by default, and they could do better there (and actually did better in the past, where there was a very visible popup asking you about this stuff).

As for "they should test this!": Right, and they do have unit tests[2].

[1] https://firefox-source-docs.mozilla.org/toolkit/components/t... [2] https://searchfox.org/mozilla-central/source/toolkit/compone...