That's... that's what I said? That's why I put "grieve" in quotes, because in the article, they used it specifically as a reaction to death. But death is probably not a concept a cat understands. They understand concepts like "absence" and "missing," but I doubt they know that things die.
This is anecdotally wrong (see below), but also doesn't make much sense on its face. Why would social animals not recognize death? A dead animal poses many dangers to a social group, so there is evolutionary pressure to recognize death. I also don't see a reason to assume that understanding death requires more intelligence than a cat has.
But aside from that, I'm anecdotally sure that cats understand death and grieve. I've seen a bonded cat search for their friend who died earlier, but the cat didn't yet know. Once the cat saw and smelled the others body, this searching stopped. They instead became lethargic for multiple weeks, in a human I'd have said depressed, and they looked for much more human contact than they had before (even when the two were separated for a couple of days).
If you're correct, why did the cat suddenly stop searching everywhere, when it didn't stop searching during previous separations? Why did it not try to "wake" their friend? Why did it eat less, play less, and search for more human affection in the following weeks, even though it never did so during previous separation?
Killing is what a cat does, left to its own devices.
You have to define what this means, because I honestly have no idea what you mean. Animals understand alive vs not alive perfectly well.
Why are you so confident that cats can understand some concepts but not death, as a predator they kill animals, so do youy think that in their mind they think that after they kill an animal that animal is just sleeping and should wake up soon?
I am wondering what a cat that acknowledges a friend died or "is sleeping the long one" is thinking if they dream about them, is that confusing for them, dead cats comming to you in a dream.
This is a very human approach to describing how a cat perceives the world.
A lot of animals kill other animals. Some of these animals don't even have brains. I don't think "killing" is a necessary or sufficient precondition for understanding death.
"is thinking if they dream about them, is that confusing for them"
I think this is an interesting question in general. Cats seem to understand the difference between dreams and reality. When my cat seems to have a nightmare and I wake her up, she calms down immediately, so she must have some understanding that her dream was not real.
So when a cat leaves a dead mouse as an offer she does or does not understand that the mouse will never wake up? I think the cats know that dead is irreversible and a cold dead cat is not sleeping.
Do you think then that no other animals understand the concept of death or you think cats are not intelligent enough like monkeys/elephants.
My own hypothesis is that cats catch prey and play with them for a while as a way of getting more training per kill. They probably find some enjoyment in it, which is likely evolved as an incentive to practice. Cats are certainly well known for their playfulness, which tends to be stronger at a young age.
I suppose both theories might be true at different times, depending on the context or the prey. I guess mice aren't particularly capable of doing any real harm to a cat.