Sure, you could look at this for example: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4892312/.

They are considered toxins because that's literally the role they play in the plants biochemistry – to defend against fungi, bacteria and animals trying to eat them – which you can learn about from many credible sources by just googling it.

The link to anticarcinogenic effects is most probably connected to hormesis and not the molecule in itself.

I don't put too much weight on research as absolute truth when it comes to lifestyle and health, since it's impossible to design a study that isolates the effects of a single molecule or group of molecules in a living organism. In this specific example, the primary thing that's valuable from a scientific standpoint is the "evidence" that such molecules have an effect on our body once they are inside our system.