To clarify, the badges awarded by Achievements are done automatically when a user meets certain criteria. They cannot be manually awarded by local admins and even Fandom's ability to do so is limited. The creator's badge is given to whomever started the wiki. The fact you and the creator decided to be "co-creators" doesn't matter as far as the system is concerned. A wiki has a single creator, end of story.
As AlexsMoonlightRose and Pafton have said, the awarding of the creator's badge was programmed with the assumption that Achievements would be enabled by default. At least a few years prior to UCP, Fandom changed that. As such, even the creators of wikis haven't been getting the badge for the past several years. Prior to UCP, Fandom was able to retroactively give the badge to the creator. However, since UCP, there has not been any confirmation that this is still possible.
And just in case anyone reading this is wondering, there has been confirmation that Fandom has suspended enabling Achievements on wikis that do not already have it. Supposedly this will be the case until Fandom decides the future of their achievement system which is currently significantly different between Fandom and Gamepedia wikis.