Another one here who is baffled by this. I initially thought, stretching charitableness to its very limits, that at the relevant time he could have been concentrating on the royal joke and the racist connotations simply didn't occur to him. But, even if that were the case, his response once he realised, was, quite simply, really 'off'. Surely anyone decent would have been totally mortified by this, and would have covered themselves with apologies - not blamed others for supposedly misinterpreting the joke. Even after being sacked, he's referring to "that ridiculous tweet" rather than "that awful, incredibly hurtful racist tweet that I will bitterly regret for the rest of my life".
Additionally, I was unaware of the Millwall connection. I agree that it's impossible to understand how he could have missed the connotations. And how could he be unaware that Meghan was mixed race? Is he really saying that, as a well-informed man and a journalist, he has never ever seen a picture of her mother?
But - I would never have had him down as a racist. I realise I may well be totally wrong about that, but I really am seriously wondering whether he was off his face or has had some sort of breakdown.