Even though he tweeted a racist photo?
The photo itself is not racist. It's a photo, of 2 humans holding a chimp dressed as a human. That's it. What makes it racist, is the context it's used in. Tweeting it to mock the parents of a mixed race child would be racist. Sending it it to David Attenborough on a birthday card wouldn't be.
But if you don't know the child is mixed race, then the context is missing. Or even if you do know, but are not sending it for that purpose, but sending it to mock the role in society of the new royal arrival, as in "even a chimp could do his job", then that's not racist either.
Context is everything, and none of us know that. People are dressing their opinions up as fact.