It is definitely true that Joyride can't make VS Code as fully extensible as Emacs is. VS Code is not designed this way. And also some extensibility from an extension need to come from an extension manifest when an extension loads, so Joyride can't declare new commands for instance. I'm guessing that is the simplest thing in Emacs.

Having, draw a rendom number out of a hat, 50% of Emacs extensibility at your REPL is fantastic, and worth pursuing. Joyride can do everything a VS Code extension can do, except the things i mentioned above, and anyone who has used some VS Code extensions knows that that is a lot.

However, the self documenting part described above is not out of reach. Most of the code building up the VS Code API is open source. Same goes for many of the extensions. Looking up and navigating to any of that code is possible. Perhaps a lot of work would need to be put in, but it is possible.