I think the idea is great, and the following comment is in no way some kind of put down because I think FCC does a wonderful job of making learning accessible.
Being as FCC is trying to look more like an editor, why not just use an editor locally? There’s nothing more authentic than using a local editor, and saving and running the programs on a users own computer. So, would it also be useful to use the code that is used for testing the projects’ code, in the challenges as well? That way, learners can learn and use all of the code (boilerplate, etc) in their own editor with live preview rather than utilising some hidden code inside the online editor that makes things work. They can also use an authentic web console for troubleshooting and exploring. And, of course, any code would be saved locally rather than have to be downloaded at the end of each challenge.