Open-source projects will generally be much easier to set up than a typical corporate codebase. A lot of effort to set up a corporate dev environment is annoying, but to an open source project it's death.
Getting familiar with the codebase is still an issue, but at least that knowledge has the potential to be useful even if you don't get the job.
This is true, but generally speaking, most companies aren't working on open-source projects like Jeff's Discourse is. Most companies that want us to do this "real" coding assignment are in corporate dev environments.
I have no doubt this process works well for Jeff's company which is open-source (as far as i'm aware), but it doesn't work so well for, say, Stripe or Kickstarter, whose codebases are NOT open source and likely have complicated dependency requirements.
Getting familiar with the codebase is still an issue, but at least that knowledge has the potential to be useful even if you don't get the job.