Looks like it's still a pre-release feature at this point.
Hot code-reloading part of the talk: https://youtu.be/7WgCt0Wooeo?t=1519
Hot code-reloading part of the talk: https://youtu.be/7WgCt0Wooeo?t=1519
It has been merged to devel branch: https://github.com/nim-lang/Nim/pull/10729