I don't know if any of you all know but they were started to be considered as natural X-digimon ever since they were considered as Red-frame cards in the Hyper Coliseum... however there are MANY instances that proves they are not X-digimon. Sangloupmon is an example, it's profile states that it existed from Genesis of Digital but at that time X-antibody never existed. Many of you don't know but the explanation of the booster pack that introduced the red frame digimon just said that the red digis are the NEW inhabitants of the NDW, not necessarily X-digimon and we know normal digimon live there too. And this is further proved by the fact that there were many blue frame Digimon too who were immune to the X-antibody in the card game. Dorumon is the only exception, as it's data is hidden so inside it's digicore that it CAN'T even evolve to Dorugamon without the X-antibody. So I hope you got what am trying to say, the very notion of natural carrier doesn't make sense, no Digimon can be natural carrier as the X-virus is an artificial phenomenon and they have to expose themselves to it to get the X-antibody. It's just that over time information about many aspects of Digimon has been lost and misconceptions arised to fill in the gaps.
Halfway through, this seems to be more about your headcanon than anything else. Red Frame cards are definitely about X-Antibodies. There are literal Red Frame Option Cards for the X-Antibody itself (Bx-6, Sx-27, Sx-107). It has nothing to do with NDW. Rather, that makes no sense. The reason X-Antibodies exist is because they live in the old Digital World plagued by the X-Program, not the NDW. The whole
Chronicle X storyline takes place in the old Digital World, for example. The non-X-Antibodies that live in the old Digital World would probably be those immune Blue Frames mentioned.
Natural carriers aren't just labelled for the cards either. The entire roster of the
Pendulum X V-Pet are X-Antibody carriers, including many natural ones. The
Digimon X V-Pets also include many natural carriers (though there are several regular Digimon as well).
The reason the natural carriers' profiles contradict the X-Program scenario is because natural carriers are treated as regular Digimon in the general "nullverse". None of the natural carriers' profiles even mention the term "X-Antibody", including Dorumon's/Alphamon's. The closest would be GranDracmon and the Death-X line mentioning "Death-X-evolution". All of the other natural carriers' profiles treat them as if they were always a part of the Digital World, similar to how many Armor Digimons' profiles try to integrate them into the natural ecosystem (like Sagittarimon being a mutation from Centalmon).
THIS 