Not sure if this is related to Codespaces or just a Linux environment, but when I log in and out when impersonating and switching between users, the bin/ember-cli -u just terminates, with no logs that point towards any errors that may have caused it. And the codespace is up-to-date with the Discourse repo.
I had to enable experimental_impersonation so I didn’t have to log out on my 2nd dev env (also on GH Codespaces).