The idea is that somehow veganism is a white person's idea and espousing that position in the presence of a person of color represents some sort of cultural oppression
People of color (in America) are 2-3x more likely to be vegan. The only way to advance this argument is to ignore reality and proceed with a weird caricature of both vegans and people of color that somehow excludes the two groups from any overlap.
So let's ignore reality and just grant the more limited claim that a white person isn't "allowed" to tell a black person that carnism is unethical. Now, then, is it also unethical for a black person to try to convince another black person that carnism is unethical? And if so, then why would it matter whose mouth it came out of? Would the color of my skin change whether the premises follow from their conclusions? It can't. That means that veganism still stands on its moral premises, and all this argument can do is say "Well veganism might be right, but you are obligated to withhold that information from minorities if you aren't one". If you admit veganism stands on its premises, this isn't a good argument because then you're saying that avoiding some sort of imaginary cultural oppression is more important than the lives of beings that can and do actually suffer, which doesn't make sense.
This is kind of a weird argument because you're saying "well veganism might be correct, but people of color are somehow too delicate to hear the truth just on the basis of their skin", as if when you make an ethical argument to a minority, it better be near a fainting couch. If veganism weren't correct then the person would then make a counterargument to the vegan, and one should respect the autonomy and intelligence of anyone one is talking with to then make such a counterargument. But are you seriously suggesting that people of color are incapable of such things or need some other layer of protection?
At best this argument saying "Don't be judgmental of people that are different from you when you talk about your positions", which is totally fine, but it's useful to bear in mind that 1. this whole "judgmental vegan" talking point is just another attempt from carnists to circle the wagons, and 2. someone being mean about an argument doesn't invalidate it.