The good thing is that the DIY people don't need to reinvent the wheel - you can just fork an existing project and shape it into what you're interested in. There's are already some bare bones implementations around that lend themselves well to building things on top of.
Of course not arguing that making it simple isn't a good thing - but the reality is that interoperability between platforms like this is a complex challenge, and even though nothing is perfect ActivityPub is doing a pretty neat job making it a reality.
Are there any good and well known examples of this at a large scale, beyond Wikipedia and it's projects?
Not that Wikipedia is not evidence in it's own right that it could work, I'm just curious.