Within the context, rule of thumb. Plants become toxic, animals adapt to it. Except humans. Instead, we adapted our food supply to be less toxic.

Fruits and berries is a different story. Seasonal so even if they're very sweet it doesn't make much of a dent over a lifetime, sweet when ripe so we (and other animals) eat them (wait a little more to get a good buzz from the alcohol), but the seeds therein are toxic and/or indigestible, we crap them out, they sit in a nice and rich growth medium somewhere else. The rest of the plant is just as toxic (or simply inedible) as other plants that don't produce fruit, so we don't eat that.

So, those plants aren't out to kill us with their sweet fruit, on the contrary their fruit has adapted to make us eat them. Eat lots of those though, we get sick. But that's a new thing with industrial agriculture. Now let's imagine a similar adaptation where plants produce fruit all year long, and we eat them all year long, and it makes us sick cuz the sugar and stuff. Well, now since those fruits have become toxic just by virtue of sheer quantity (just as it is with industrial agriculture), species that get sick from that die off, those that adapt somehow survive. This sheer quantity then becomes the determinant of subsequent parallel adaptation. If the species that get sick from all that fruit die off, the plants no longer have a means to reproduce, they die off too. Either the plants adapt by producing fruit less often or fruit that's less toxic (get rid of glucose/fructose, use the stuff found in stevia instead), or animals adapt to eat less fruit less often or adapt some immunity. Think of it as the natural selection of low-carb. When we go low-carb today, we are in fact artificially selecting ourselves to survive our current food environment.

For adaptations other than becoming toxic, it's still the same principle of parallel adaptation from plants and the animals that eat them.

Anyways, it's just a side point to illustrate how dumb that phyto-nutrient stuff is.