Even ignoring the Hackney incident, the ref made a number of other very strange decisions yesterday. It wasn't a good performance from him.
The problem is it "looks" bad. Hackney plays the ball before the Cov player arrives and is bringing his right foot down which is why his studs look like they're showing. The Cov player makes the most of minimal contact and cons the ref.
Shouldn't have been a yellow and Hackney was unfortunate, but having already been booked he should have been more careful. His previous early bookings have all been rather pointless too. Someone needs to have a word with him. It's almost as if he isn't willing to trust the defence at times.