First, his usage wasn’t racist. He applied the word to himself, as a descriptor to be proud of.
Second, it’s contrived, straw-man nonsense to suggest Adams was equating his experiences in his era with slavery-era US. The word didn’t die out with slavery. It has a modern usage as an indicator of social prejudice in a post-WWII context. And there are social parallels. Civil rights marchers in the 1960s sang the same songs as black civil rights protestors in the US.
During her 1969 trip to the US, Bernadette Devlin had this to say: “My people — the people who knew about oppression, discrimination, prejudice, poverty and the frustration and despair that they produce — were not Irish Americans.
“They were black, Puerto Ricans, Chicanos. And those who were supposed to be ‘my people’, the Irish Americans who knew about English misrule and the Famine and supported the civil rights movement at home, and knew that Partition and England were the cause of the problem, looked and sounded to me like Orangemen. They said exactly the same things about blacks that the loyalists said about us at home. In New York I was given the key to the city by the mayor, an honour not to be sneezed at. I gave it to the Black Panthers.”
Third, the white middle class post-hoc adulation of Mandela doesn’t alter the fact that, for most of his life, respectable white people despised Mandela. To Thatcher, he was a “terrorist”. Frank Millar dismissed him as a “black Provo”.
Finally, following Adams’ tweet, the papers are full of outraged “white saviours” who seemingly are comfortable to act as spokespersons for black people.
Presumably, no irony is intended.