Good question. I rarely look at other add-ons, so I don't know whether there is some official, Google approved way to promote your add-on / app as approved by Google.
I did apply to become a Google Partner, and was initially accepted, but I am not following through with pursuing that. And I don't even really know what they are offering. I know that the Google Partner people basically refuse to answer any questions, unless you provide personal information, agree to the Privacy Policy, and make an appointment with them.
What I'm saying is, if you become a Google Partner, it's probably acceptable to advertise yourself as a Google Partner. I guess the only issue would be if you advertised as being verified and were not. But that would probably become quickly apparent by anyone installing the add-on.
I'm not the person to give an official answer, and I'd like to know also.