This position is put forth by Matt Dillahunty. So the basic idea here is that we have moral imperatives and moral virtues. Moral imperatives would be immoral if we break, but moral virtues are not immoral if we don't do them. For instance, it's wrong to kill someone, but it's not wrong to skip donating to a charity. It makes you a better person to donate, but it's not a true moral imperative.
Veganism, under this argument, is like donating to charity. So veganism is good, but that doesn't make carnism bad.
Let's assume that whoever is promoting this theory would define taking pleasure in human murder (including of infants and the mentally handicapped) as immoral, along with dog fighting and other similar forms of deliberate animal torture. So under this system, these things all fall under the first category (immoral if committed), whereas carnism falls under the second (not immoral if committed).
Then all that's happened is the arbitrary separation of different types of abuse of sentient beings for one's own entertainment. Any reason for this separation is already covered by any of the other arguments. Are you separating based on species and their intelligence? Are you lying to yourself that more narrow definitions of torture is the only abuse? Is it based on an irrelevant non-sequitur, such as our bodies or that Disney movie with that theme song?
Whatever it is, then that is the actual justification, and this whole argument is just then a framing mechanism to make that argument sound more complicated than it is.
So some forms of deriving pleasure from sentient-being abuse are immoral, some aren't, and it happens that the most convenient choice is where the separation lies (This isn't the first time). It then gives the impression of it being an arbitrary speaker bias. What would be needed to avoid that and promote this claim into a serious argument is:
Until such arguments arise, this is, at best, a framing mechanism for other carnist claims and at worst, an easy ticket out of having to provide a serious justification for carnism.