By moving the pivot foot backward off the pitching rubber before lifting the lead foot, the pitcher completed a legal disengagement under Official Baseball Rules (OBR).

Rule 5.07(a) Legal Pitching Delivery (“Set Position” Comment)

Rule 6.02(a) Penalties for Mass Infractions (The Balk Rule)

Why This Is Legal:
Becoming an Infielder: The moment the pivot foot touches the ground completely behind the rubber, the pitcher officially drops their status as a pitcher and becomes a regular infielder.

Freedom of Movement:
Once the pitcher is an infielder, they can legally drop their hands, change their body position, or abort the play.

The Key Factor:
As long as the pitcher did not make a sudden, deceptive motion to home plate (like starting the actual forward drive of a pitch) before that pivot foot cleared the rubber, stepping off backward completely resets the play.