Well, they are not zener diodes , or TVS diodes, and they are in a parallel-anti-parallel configuration. They will provide no overvoltage/transient protection. All they provide is a voltage drop of the forward voltage of the diode.
For them to provide any transient clamping, they would need to be zener/TVS diodes, to ground for positive transients, or to the positive rail for negative transients.
In this case, all they are doing it providing maybe a tad bit of microstepping performance improvement , and as you said, as a standoff.