I don't know the mechanisms, but plants certainly produce various chemicals,
anti bacterial, anti fungal, anti viral, insecticides, feeding inhibitors, even outright toxins and poisons.
Many of these have long been extracted by us as medicines spices, or horticulture treatments.
I think there is some evidence that plants can link up via roots and send chemical messages to each other re pest attack.
If these finger lime extracts prove successful against HLB, I hope it sets a trend for further research along these lines.