I read through it. I'm kind of indifferent. It's probably a better way to interface with certain 3rd party code bases, but I don't see it being universally better than using libraries.
I think this sidecar pattern is trying to make a certain class of dependencies a configuration problem instead of a coding problem, but I'm not convinced it's going to work. Its too easy to imagine someone bumping a sidecar version, then needing to update their business logic to account for some minor breaking change in the sidecar's API.
The thing that seems really cool to me is that you can have dedicated UIs for each sidecar. That seems like the killer feature that makes the whole thing worth while. There's no reason you couldn't do the same thing with an in-process dependency, but the fact that this is a real think that's working now is very cool.