More like: This plant has no insect parasites, it must be special somehow, let's try and use it in different, increasingly "close approach" ways. I mean, humans must have figured quite early that on average, inhaling smoke of a poisonous plant has a fraction of the effect of chewing the same. Someone gets really sick after chewing some leaves, their family burns the rest of the "crop" and they get high.

The cool thing about tea leaves is that the caffeine is stored in crystals that are not particularly soluble. There's an organelle that contains an enzyme that breaks the crystals down into a soluble form.

There are a couple varieties of tea that are actually fermented, but for most teas it's a misnomer. You aren't causing fermentation, just autolysis (latin: self digestion), which is more akin to the malting phase of beer production. Black tea is left to process longer, while green tea is interrupted sooner. The switch is turned on by bruising the leaves, and off by desiccating them.

From a caterpillar's perspective, a tea leaf is booby trapped. Waiting for mandibles to mix the ingredients and create the insecticide.