Here's a short version of the "carnism" theory:
We're all conditioned to exploit animals by an "invisible" ideology called "carnism." We need to expose this "invisible" ideology.
But the ideology that accounts for our exploitation of animals is not "invisible." That ideology is the animal welfare position that has been around for 200+ years. The animal welfare position—that we may use animals for human purposes as long as we treat them "humanely"—is accepted by most people in the general public and, in one form or another, is explicitly promoted by many large animal organizations.
The anti-carnists claim to reject the idea that it is morally acceptable to kill and use animals as long as we treat them "humanely."
But the anti-carnists also claim that it is *not* acceptable to reject the animal welfare position because some people (but not all) who promote it are vegans and really want to see people go vegan eventually but they think that this will only happen by promoting welfare reforms. The anti-carnists also think that it is "divisive" to criticise animal welfare since so many animal advocates embrace it.
The anti-carnists maintain that those who promote welfare reform and those who argue that we cannot justify any animal use and regard veganism as the moral baseline of the animal rights movement are the same. Any distinction between these positions is a "myth."
Therefore, we have to continue to support the animal welfare position, which explains why people think it’s morally acceptable to exploit animals, and we must instead look for an "invisible" ideology because the clearly visible one is one that we aren’t allowed to criticise and it’s no different from the rights position anyway.
Apart from being completely incoherent, the "carnism" theory is just another transparent attempt to stop people criticising the animal welfare approach to animal ethics.