The last couple of years have been chaotic for many of us.
Unfortunately this area of work isn’t something that I am directly focused on at present. Had there been more interest and concrete use cases back when I posted the demo, I may have been able to fold them into the work I was doing then.
I am however working on interacting with remote data sources via HTTP requests from wikitext, along with manipulating the resultant data. This underpins the recent WebDAV work as well. All the primitives needed to enable the workflow you describe already exist but need further testing and improvement. Once this work is mature enough, it should provide the basic wikitext affordances to be able to fetch tiddlers from a remote TiddlyWiki and load them into the current wiki, all from wikitext.
Worth noting here also is that TiddlyHost provides access to just the JSON of the actual wiki content, which would make the loading of remote tiddlers a lot less data intensive than having to load complete TiddlyWiki files in the background, see for example Simon's Tiddlyhost site #1