That's a really good question
In some sense IndieWeb is just having a website of your own on a domain of your own
So e.g. flancia.org is already IndieWeb
Then you get the webmentions stuff for interacting peer-to-peer between sites
I'd say a digital garden is very much IndieWeb
And I guess I think of Agora on one level as a garden aggregator.
Agora is kind of like for gardens what micro.blog is for streams.
micro.blog uses a lots of the IndieWeb protocols, but makes it a lot easier for non-developers to have one.
Like you literally just set up an account.