It’s not bloody reasonable. He is fully aware of the racist link - he has admitted this himself. So either he deliberately intended to be racist, or his primary purpose was something else and he just didn’t give a shit about it also looking racist. Both of those attitudes are racist!
There's a third option. He is aware of the racist link, he is aware MM is mixed race, but at the time he posted his joke, neither of those even occurred to him, because he thought he was making a funny gag about the royals being a performing circus.
When it was pointed out, he suddenly realised his oversight, and deleted the tweet.
That might make him an idiot, but not a racist.
Then again, he might well be a racist.
None of us know. Sadly, only a few of us know we don't know. Most people seem to like to think they do know.