As mentioned by others above,
They're likely thinking of things akin to fear of the unknown and ways to avoid death in a time before we had human civilization as we know now (i.e. when we would be annihilated by other species because we didnt have a proper way to fight back). Its true that recognizing things that are dangerous helped us survive, thats just pattern recognition. However at a certain point it diverges into (harmful) stereotyping and discrimination, which is obviously not the best.
By "The world is actively trying to kill them" they are referring to the things in the world, such as poisonous plants or predators. A sorta "everythings out to get you".
Dear god mangadex comment sections can be a hellscape at times. Glad to have missed most of this.
It's not really even that complex. The "us vs them" or "in and out group" mentality is deeply rooted in basically every living thing that forms groups. It's not really pattern recognition, because pattern recognition is more logical, accurate, and helpful. This is full of biases we are both aware of and unaware of, superficial, and actively harmful to humanity (even in the stone ages). It's more fear and paranoia than anything. That's just it though, humans are irrational and self-interested. We fixate on superficial similarities and hyperfixate on superficial differences.
The most basic example (without using humans) is airplane crashes. Actual data shows that flying is the safest way to travel. It's far, far safer than driving a car. But people are more fearful of flying and it feels like the news has stories of bad plane crashes more often than car crashes. But none of that's real. That's not pattern recognition, it's the fact that Plane travel is other'd. You don't do it everyday and people don't have planes parked in their drive way. People encounter flying less so they are more weary of it. People know less about the act of flying a plane (they don't fly planes), so they are more weary about it. In fact it's that they don't see the pattern that makes people more predisposed to fear flying. People fade news about car crashes into the background. It happens so frequently and it's using a vehicle that's so common, that no one bats an eyelash when it happens. Plane crashes are less common and so we notice more when they happen and the fact that the vehicle itself is less common makes us more suspicious about their safety. It's not about what's safe vs what's harmful to us; it's about what we're familiar with.
Saying the world is out to get us is the dumbest BS. The world isn't out to get anyone. The world doesn't give a crap, but more than that the world is a freaking miracle that enables life on our planet. A fact that isn't easily replicated. Yeah, there's poisonous plants, but there's also medicinal plants and herbs. However, beyond that none of those plants will actively try to harm anyone. They don't have legs. The world isn't out to get anyone specifically or a species. All things are just trying to protect themselves against being eaten. It's not that things have diverged, it's that nothing has changed. The core is fear. We form our groups and then everything else is the enemy. It doesn't matter if people inside the group harms us because we spend everyday with them. We'll punish them and then brush it off. Any outsider however is kill on sight. Not because we sat down with them, spent years in their group, and then ran the numbers about if they pass a percent chance to harm us. Because they are in a different group from us and unknowns bring danger.
In society & civilization this manifests differently for so many different reasons and in so many different ways and they'd all really require sociological and psychological studying. It is interesting study. For in & out groups to be such a strong phenomena, it has to be that there's a deep tribal culture in our species regions. Likely it's just that humans couldn't survive on their own. Regardless they formed in and out groups and we haven't really been able to grow past that.