Last week if you told me you had "Trump accuses Kamala Harris of racefaking in front of an audience of black journalists" on your bingo card, I would've chastised you for an incredibly poor-taste joke, but apparently you can mark that square today
So, something to understand is that he does not listen to his advisors or follow the strategy they set out for him, almost ever. His campaign strategists and advisors have been pretty staunchly pushing the GOP and the public-facing side of the campaign to avoid attacks on the basis of race or gender, because it loses them poll numbers and gains her support. This hasn't stopped him, or any of the other Republicans, they just can't help themselves, but his advisors are practically begging him to cut this shit out. The other important thing to understand about this event is that the appearance was cut short because his own campaign requested they end it, that's how bad a job he did following his advisors' strategy
This panel was set for an hour, his advisers had to cut the panel short and pull him away because he was so bad during the whole thing.
Also, if you watch this video, the FIRST THING he's going to do is start complaining about how THEY made him wait to avoid answering the interviewer's first question and I just want to set this straight: The ACTUAL reason it's start was delayed is he spent, like 30 MINUTES trying to browbeat them into A Promise Not to Do Live Fact-Checking of anything he said.