It looks as if the officer has fibbed to protect himself, and has been caught out. That's a no-no, undermines honesty and integrity, and was quite likely the greater contributor to his sacking that the excessive force. If he'd put his hands in the air, coughed it and prostrated himself in apology he just might have kept his job, but one a copper is caught fibbing that's it, adios amigo, unless your NPCC Freemasonry level of rank.
It is possible (but again, just speculation) that the officer was already on the radar of the local professional standards department, so its possible they may have been on a hair trigger, just waiting to have him for something. Might not have taken much persuading at all.