It could be understood that I could still wear mixers' signatures as long as they don't contain a URL and my username isn't mixerABCD.
Yes you can. As long as it doesn't have any links. And in turn, whoever is paying him to do so contacted him outside the forum.
Wouldn't that still violate the ad part though? I don't know how you would verify if somebody is being paid or not from this other than trusting what they said. Unless you track his wallet address etc, which can take time and it is probably easier to just ban any mixer sig. It doesn't matter where the user is being contacted since the promotion is still happening here. CMIIW.