The carnist throws this phrase out as though it holds meaning. What it means is a form of moral nihilism that states that it doesn't matter whether you buy animal corpses to eat or plants to eat, they were both made from the capitalist exploitation of the working class and therefore are equally unethical.
It's a sign that there's a lack of clear thinking when you're just repeating a phrase. Consider almost any other argument on this list, it has a lot of different phrasings with synonyms, or rearrangements, etc. Why is it that the sequence of words "no ethical consumption under capitalism" is the universal expression of this vacuous sentiment? Interesting to note.
If you do anything ever, and you live in a capitalist country, and you bought either food to live while you do a thing or a tool to do it with, and therefore now everything ever is morally justified, because you can always deflect that the original consumption as being unethical. "Should I use this hacksaw to build a shed for my neighbor or murder them? Doesn't matter! No ethical consumption under capitalism, hyuck!". But surely the people proposing this nonsense don't believe it. It's just deflection.
By buying meat, you're increasing the amount of consumption of food to feed the livestock. So even if you blanket cover all consumption under this umbrella, shouldn't you still choose the smallest amount of unethical consumption?
Many liberals boycott Chik-fil-A on account of their religious conservatism. Why is it all of a sudden the default liberal position that any consumption under capitalism is equally wrong?
But it's more infuriating because it's thinly disguised as pseudo-intellectualism and it's highly self-congratulatory. "Oh, I'm fighting the good fight against capitalism by indulging in animal products. This McDonald's hamburger is how I'm fighting the man".