The notification is currently part of a test, so not all users are seeing it. It’s not a permanent part of AMO yet. We want to gauge the reaction users have to it and decide if it should be kept, modified, or dropped.
Having said that, the message is accurate. Most extensions on the site aren’t being regularly code-reviewed, so we can’t guarantee their safety. We believe this is something users should understand. That a developer has been contributing for X years, has positive reviews, or has provided ID to us is not enough to assess safety (though it helps). We’ve had incidents in the past with long-standing add-ons and “reputable” developers. Additionally, having the ability to ban after the fact doesn’t help users who have already been impacted by a bad add-on.
We’re trying to draw a line between add-ons that are being closely monitored and those that aren’t. At the moment that’s limited to a small set of Recommended extensions, but that can change in the future depending on the success of the program.