Garuda is an Arch distro that creates a system snapshot every time you upgrade. That way, if the upgrade breaks something, you can roll back to a previous, stable system.
They probably already know how to use Discord, so they think that it's easier to migrate their community to a service that they already understand vs. researching an alternative, learning how it works, and then trying to migrate their community.
Also, a lot of their members are probably on Discord already, so you don't have to try to convince them to sign up for a new service on top of everything else I mentioned earlier.
Humans are moral agents, though. Just because something happens in nature, that doesn't make it okay. There are lots of examples of rape among wild animals, but that doesn't make it okay for humans to do it.
A lot of vegans are concerned about climate change, too, but it's really tangential to the philosophy. Veganism came out of the animal rights movement, so it's really concerned with exploitation and suffering. If there were no environmental issues with animal products, vegans would still be vegans.
It isn't a perfect analogy. I doubt that any analogy is. I regard defederation as an advanced topic, though, and it isn't necessary to understand it to grasp the basics.
We could sit here and speculate about what makes sense to the average person all day, but at the end of the day it wouldn't amount to anything without evidence to back it up...user studies or something like that.
I think the point is to have the freedom available. Most people are going to get their email from GMail, but you have the freedom to get it somewhere else if you want to, and you can still send to and receive email from people using GMail. You can even roll your own mail server.
Or you take system snapshots so you can roll back when an update breaks.