I don’t know this particular story, but there are two possible (and potentially overlapping) biological explanations here.
-
One possibility is that the plants have lost the capacity to make the chemical, due to lack of selective pressure on its continued production. This is a standard, classic Darwinian evolutionary process. In the presence of herbivores, production is selected for, or perhaps more likely, absence of production is selected out of the population (via the guts of herbivores). Without the herbivores, the occasional variant that can’t make the chemical is not selected out of the population. Then, more interestingly, the story sounds like one in which the non-producers took over the population. If that is the case, then the most likely explanation is that there is a cost to making the chemical, and so in the absence of herbivores, its production turns into a burden instead of an advantage. The plants that don’t make the chemical enjoy a slight advantage and take over.
-
The other possibility is that production of the chemical is turned on in the presence of herbivory (or in the presence of the herbivores themselves) so that it is only employed when needed. There are cool interesting examples of this kind of interspecies interaction in both plants and animals. In fact, for example, plants emit chemical signals at different times of day (in response to light) and detect and respond to vibrations induced by leaves being munched.
In neither case does the plant need to “know” anything.