How are the Scala dei Turchi staircases formed?
There's no need to invoke stretching here. This is just a simple case of dipping sedimentary beds which have been eroded by the sea.

The stepped geometry may be caused by small variations in the hardness of the beds such that the harder beds weather out more prominently and create the steps.

Alternatively, at least some of the larger steps may be wave cut terraces formed during periods of stable sea level. In the background on the left of the picture you can see two large terraces separated by a ramp which look very much like wave cut terraces. Such terraces are well known from central Greece, where they have been uplifted by activity on faults and I would speculate that a similar process has occurred here.