This argument is a claim that carnists don't actually kill the animals, they are simply paying for meat
When you go to a store and purchase e.g. a bag of frozen chicken, you advance the delivery of the next box of frozen animal corpses by one bag. The supplier orders another box one bag earlier, and therefore another bag's worth of chickens is trucked into the facility. When you buy animal products, you are still the cause of the animal dying, it's just pinballed through this system.
No one breaks down an action into multiple steps and then claims that this somehow changes the ethical nature of their actions. e.g. "I didn't kill him, I just squeezed the trigger of the gun pointed at his head. That caused the firing pin to be dropped on the prime of the bullet, which caused gunpowder to ignite, which caused.... .... so anyway all I'm guilty of is dropping a firing pin!" That's just absurd. You increased the demand for animal products. That caused more animals to die
In no other context is paying for something ok when the action is illegal. Fencing stolen items is immoral because stealing is immoral, and you are feeding the thief. Hiring a hit man is immoral because you're paying someone to do something immoral. There's nowhere else in life where payment somehow launders the action ethically.