Sprint 0 is just a team establishing what they need in place so that they can start working. It makes perfect sense. Practically speaking, it is the establishment among the team of “how is this thing going to work? — what are the big pieces and how will they fit together?” To start coding right off the bat is idiotic.

Regarding a hardening sprint, that is a symptom of not spending enough time in sprint 0 to set up the tooling and automation, so that the team can produce potentially releasable increments on a regular basis.