Well for one thing, I don't think the Zone is as popular as you think it is. It may well be one of their best-selling discs, but they have others that out-sell it by a large margin. Avenger family, Buzz family, Force, Challenger, Ringer...I guarantee at least all those sell better than the Zone.
Second point, the Zone is already pretty shallow. Putting the groove top on it would, I would imagine, make it feel very funky. Now I've only seen the promo pics of the Ringer groove top and it does look, as you stated, more like a Thumbtrac than the standard GT of the Banger. So, if you and I are both correct, it could be possible they are testing the waters to see if that would sell with the Zone or even other putters like the Challenger.
Last point, the GT is really not a common release for Discraft. The Banger GT is, of course, a stock model. But other than that, all they've ever released is the Buzzz-GT for special fundraisers and now the Ringer GT which seems to me like it is also not going to be a stock release. I actually know two people who love the Buzzz GT and use them as their primary mids, so they aren't unpopular versions of the mold, just not common. Last year when Infinite Discs was a major Ledgestone sponsor, they got to choose their own limited Ledgestone disc. I suggested a Glo Z Buzzz OS GT. They ended up going with a CryZtal Buzzz GT and sold them all, but it didnt seem like my suggestion was outside of Discraft's capabilities.
TL;DR: Zone isn't that big of a seller, Ringer GT may be a trial of a different type of GT which can be used on the Zone, aside from the Banger GT most GT discs are limited releases, it may not be outside the realm of possibility given all the Ledgestone limited discs