You say he wouldn’t have had time to think about calling the racist card but he certainly had time to think about launching Firmino into the crowd!?
I honestly believe that he thought he was getting sent off, panicked, heard Firmino shout something, and played the race card. He probably regretted it immediately but once it was said it was said.
Just my opinion and I could be totally wrong. He could have said it in all honesty even though nobody else in the vicinity heard it and lip reading experts have confirmed nothing even similar came out of Firmino’s mouth.
Anyway, let’s move on.