IIRC I think by rule the field ref should have given a straight red by rule if no attempt on the ball. I could be wrong. If that is the rule, then he could have been correcting his initial call. Im not sure what the protocol is there, but Im giving him an out.
Also, Martinez should have been booked for shouting down and interrogating the 5th official. Disgusting sportsmanship, but every team needs a player like that and Arsenal have not one.