The difference between carbon offsetting and meat offsetting is that carbon offsetting doesn’t involve causing harms, while meat offsetting does.
Most people would consider it immoral to murder someone for reasons of personal convenience, even if you make up for it by donating to a ‘murder offset’, such as, let’s say, a police department. MacAskill is saying that ‘animal murder’ offsetting is like this, because you are causing harm to animals, then attempting to ‘make up for it’ by helping other animals. Climate offsets are different because the offset prevents the harm from occurring in the first place.
Indeed, murder offsets would be okay from a purely consequentialist perspective. But this is not the trolley problem, for the reason that Telofy explains very well in his second paragraph above. Namely, the harmful act that you are tempted to commit is not required in order to achieve the good outcome.
Regarding your first paragraph: most people would consider it unethical to murder someone for reasons of personal convenience, even if you donated to a ‘murder offset’ organization such as, I don’t know, let’s say police departments. MacAskill is saying that ‘animal murder’ offsets are unethical in this same way. Namely, you are committing an immoral act—killing an animal—then saving some other animals to ‘make up for it’. Climate offsets are different because the harm is never caused in this case.
Regarding your last paragraph: This is a nice example, but it will fail if your company might modulate the amount of food that it buys in the future based on how much gets eaten. For example, if they consistently have a bunch of leftover chicken, they might try to save some money by purchasing less chicken next time. If this is possible, then there is a reason not to eat the free chicken.