The first immigrants to Australia didn’t come voluntarily either: they were convicts, and transportation to Australia was considered a worse punishment than hanging them. Today Australians are as proud of their convict ancestors as Americans are whose forebears came over on the Mayflower in 1620. The more heinous the crime, the better. We still call them immigrants, because they came here from overseas.
I think you are being far too precious in your political correctness to get upset over someone referring to Slaves as involuntary immigrants, because like it or not, it is literally true.
I could add that white guilt over slavery can be overdone: remember that at the same time a vastly greater number of slaves were owned in Africa and the Middle East by Black or Arab owners, and that these included a great number of white slaves, male and female. Slavery is a beastly institution, but it is quite false to imply that all slavers were white or that all slaves were black.
These untruths do not help further the reconciliation between the races that is needed for a civil society in the future.