Parts of the flap got burned and/or taken away with the airflow but majority of the control surface was left intact leaving enough manouverability to continue the mission. The electric drive obviously survived the reentry and continued operation of the flap and the control software was probably able to adapt to the partial loss of available aerodynamic surface.

In terms of test data this sequence of events was way better than nominal mission flight. Now they know how much surface then can lose and still have a successful touchdown.