I can't tell you how many times I've read posts about Namor appearing in the MCU. A couple of years ago, SYFY even reported that Namor had a movie coming out when Marvel was filming in Hawaii. Turned out it was Triton in the Inhumans. The only real news about a Subby movie was reported on the trade sites, and that was mostly development news, i.e. at Universal and the director changes.
Recently there's only been one description that mentions Atlantis specifically, and they mention Namor's father going to Antarctica looking for vibranium. This was first posted on an unofficial Disney centric site that claimed to have gotten the info from Production Weekly. Those subscriptions are $675 a year. Maybe they are coughing up that kind of dough, but I kind of doubt it. Most of these sites just parrot what other sites are posting / making up or claim to have inside sources -- who have been wrong more often than not.
Of course, I could be wrong. They clearly are doing something with Huerta. But I don't think it is in the best interest of either Black Panther or Namor fans for it to happen. Just like the comics, they aren't going to let Namor die or lose badly. And without T'challa, I doubt the rest of the cast is going to hold their own against Namor. He's a scene stealer.