Its probably true of most animals. I raised a couple of pigs and we intentionally never fed them any meat, even though they'd probably eat it, because we didnt want them to get a taste for it
I suspect it is not just when fruit is scarce many if not most primates are omnivores. Fruit is likely easier than meat most of the time but meat contains dense nutrients. Macaques e.g crab eating macaques routinely eat meat.
I don't feel like the article is lacking for that. It's evolution at work. There could be monkeys eating animals that can properly digest them due to some mutation or unique gut biomes. Actually eating the animals is an important first step. Monkeys that usually eat fruits are hunting and eating animals when food is scarce, that's an article worthy observation.
Is that not the case in Central/South America?
https://www.startribune.com/minnesota-wolves-may-be-helping-...
https://www.ucdavis.edu/climate/news/carnivorous-feeding-squ...