While somewhat true, essentially the question of 20 amino acids still holds and trying to answer it is still meaningful (and in the same way article addresses it). Consider this: how old are these plant insecticides (or whatever they are)? Maybe some 150 mln years old? (I'm only assuming, of course, but it seems like a plausible assumption.) Well, the life on Earth itself is about 3.5 bln years old, and as far as we know it mostly utilizes the same set of 20 proteinogenic amino acids it did back then. It is only natural that given time, constantly mutating and evolving organisms would eventually learn to produce whatever amino acids possible, if it is useful (and being insecticide is obviously useful, if you are a plant, and for reasons you mention yourself it is natural to use as insecticide some compound that is not normally utilized in the other organism). The question is how did it occur that these amino acids weren't present during immune system development? Why it was always the same 20?