It's a frequent "let's play absurd" argument from meat eaters that plants have a central nervous system, too, and suffer and that they are being nice to plants by not eating meat.
I have NEVER heard this said outside of joking. If someone has actually tried to seriously argue a plant CNS with you, kick them in a soft spot - for me.
However, plants != meat. You come close to realizing this but quickly step away from the edge. Meat is "inefficient" because it is a degree removed from the original energy source. That is, the sun jaunts on down and gets captured by a plant to be used as energy to convert local material into being more plants. We can now eat that plant OR we can feed that plant to an animal which will then use the energy and materials to make more animal. The extra step is subject is lossiness, just as you would expect.
But we don't do this lossy conversion just because we're arbitrary jackasses. We do it because plants != meat. You can try to slam down as much quinoa and avocado as you want, but it's not the same nutrients, not even the same protein nor fat. A slab of fatty cold water fish and some chopped liver once a week cannot be replaced by plants. Only with modern globalization has this plant/meat replacement become even a remotely possible thing (unless you can point to a single location where all of these fatty and proteiny plants grow in one place), even then it does not come highly recommended.