Similar rules apply for the use of the WordPress logo, especially if it's used as part of another logo. In the case I found for a prominent plugin, by a prominent community member, I contacted them privately with a link to the logo rules. They checked it, confirmed it, then updated their plugin logo within a day or two. I contacted them a few weeks later to remind them about the plugin .org repo cover image, but they quickly updated that as well.

Apparently one or two plugin users noticed the logo change, but otherwise no big drama was made about it, which was the best way forward in this case.