I assumed it was deliberate. In fact, I thought it was a deliberate attempt at a poetic sentence (partly due to the comma use):
In that time between lunch and dinner,
when footfall has fallen,
the restaurant quietens,
until it’s empty.
OT, but can the verb "quieten" be used to mean "become quieter" without "down"? I wasn't sure, I don't hear that verb very often, it's more BE, right? "Quiet" works that way in AE, it is used to mean "become quieter", usually with "down" but sometimes without.