It's not really custom. It's industry standard protocols for connecting independent (one might say "autonomous") networks.

Tailscale is this on easy mode, of course. There's a blog post by apenwarr somewhere that I can't find right now that lays out the fundamental thesis of Tailscale and its very similar to these folks' manifesto.

The custom jiggery pokery is the point. This is like telling someone to throw away their hobby car they work on over the weekends and replace it with a new electric vehicle because it'll, "Just work." The configuring things, understanding the infrastructure, that is _the point_.