Doing this on a web application isn't really that impressive and could have been done in almost any language.
The awesome thing is that this could have worked with an installed desktop software, without downloading new binaries and installing them, simply having the application download a tiny text file with the source code change to input into the REPL.
No recompiling, no pushing huge binaries, no hassle.
I had to implement a big cumbersome system to enable automatic incremental live updates on our client software, so users will always use the latest version. Using Lisp just makes it a breeze.