Plants do have enzyme systems that react to stimulii. More ancient plants (orchids, grasses) are more mobile than more modern ones trees, and can actually move and colonize. When a plant is damaged (fir instance when an apple is bitten into) within minutes the enzyme system reacts and the apple begins to produce chemicals which would eventually grow a "scab" if the hole is not too big, or the eater consumes them. (You can see this yourself, leave an apple with a small bite out and the bitten part will change colors. This is an enzyme action caused by "damage." It that apple was still on the tree it might heal. Many plants have the ability to self heal (trees do) and plants have many more abilities based on enzymes including reproduction (which gives us fruit). At no time in this does a plant feel pain. enzyme systems can, and do exist without/ independently of a central nervous system.
I think what people like this forget is that chemicals and also "organs" are the cause of feelings, and sensations. Creatures need certain physiological apparatus to experience different things. Some creature that does not have nerves will not feel anything. Nerves are easy to find and made of certain tissue, so if they had nerves, people would know about it by now. We know things like what a dog can taste or if certain animals are colorblind by their anatomy, not by asking them. certain animals actually do not feel as much as others based on their nerves.
We know also that animals like cows, pigs, horses, dogs etc do feel anxiety, fear, "love," because they have the brain chemistry and hormone producing organs needed to feel such emotions. So killing an animal is accompanied by anxiety, fear, probably frustration, and finally pain very much like human feelings.
Finally, those people have probably never had an original thought in their lives which was not planted by someone else and delivered by wrote, like a good little puppet.
I personally understand that you can get a larger muscle by eating protein from animals, it is true. 'although that may appeal aesthetically to some people, I know there is a large part of the population that those thick necked body builders seem bizarre. I prefer the more slender obviously ripped with low body fat sort of body. It seems to be more natural and also seems more healthy and youthful.