Some animals are better than others for eating and spreading seeds. If a plant could choose it might prefer birds eat its fruit, especially over animals with molars that crack the seeds. Not coincidentally, the active ingredient in hot peppers, capsaicin, impacts mammals but not birds.
It might very well be coincidental, because the hot compound also has anti-fungal properties. So the primary evolutionary benefit might have been that, not selection of animals eating the seeds.
Saying it's not coincidental does not mean it's planned. The whole point is that random variations that are advantageous are the ones that survive; that's not coincidence, it's consequence.
Evolution does not plan, but it does react. Capsicum seeds eaten by birds have a reproductive advantage over seeds eaten by mammals, so a defense chemical in the fruit that discourages consumption by mammals without affecting birds is not a coincidence.
This is why it is necessary to stretch bird nets over research-breeding beds for strawberries, to get varieties more attractive to paying human customers. Otherwise, the evolutionary pressure to be attractive to birds still heavily affects the results.
There's different ways to look at it. Evolution doesn't plan, and the development of this or that compound is driven by random mutation- but it reliably exhibits all kinds of emergent behaviors, like evolutionary arms races, convergent evolution, and so forth. Like many other systems with feedback, given a set of constraints it reliably trends towards a local optima.
It's not coincidental that the adaptation wasn't lost. Mutation and genetic drift may be thought of as a random process, but natural selection is much less so.
Evolution doesn't plan, no, but when you're looking at survivorship, can you still call it a coincidence?
Co-evolution uses happy accidents all the time, but then some of those accidents get baked in, and what's left over is the result of that accident.
Peppers became more likely to be spread by birds because they became unsavory to mammals. Then what happened? And then what happened? And now we had pre-agricultural peppers, and now we have ... something else.