OK, experimenting with this example, I attempted to import a simple Racket web app (https://github.com/jarcane/RantStack). I've provided links to the REPLs too for both attempts.

attempt 1) I was able to import a Racket app from Github, added a similar nix config to it as described in the docs, but it would not run as it says nix-shell is not present. Apparently a repl needs to be blessed some how as a Nix instance to work, but there's no option to select that when importing or creating a repl. URL: https://replit.com/@jarcane/RantStack

attempt 2) Next I tried just forking your template, and literally just copy-pasting the code from my app (it's a single file anyway) into the main.rkt. This runs the app ... but I can't actually access it, because apparently the ports aren't being forwarded. Going to the URL as described in the repl.it docs just gives me an eternal "Repl waking up" screen that loads forever, but never resolves. URL: https://replit.com/@jarcane/racket