This argument is generally used by people to defend bad institutions. I've heard Confederate apologists makes the exact same argument you have here ("Look at the Union treatment of Native Americans. All sides in the Civil War were immoral.").
There are gradations of bad behavior. Treating bad behavior as if it's all the same just encourages a race to the bottom.
Did you look into that claim at all? Michael Twaddle's "The Ending of Slavery in Buganda" seems to be one of the central writings about the end of the slave trade there. It seems that the connection to the Indian Ocean slave trade was through trading with Zanzibar. British colonial administrator Frederick Lugard ended the slave trade with Zanzibar, and freed the slaves there. I've looked at other sources as well, but from everything I can find that seem to be the extent that Buganda "helped end" Indian Ocean Slavery (there British colonial administrator helped end it).
If you read historical research on the ending of slavery in Buganda, it's similar to what I said. It was pushed by the British, often against intense local opposition.
The abolition of slavery in Africa is interesting to read about. It was mostly European colonial powers who were trying to end the practice of slavery in Africa, and they often faced a lot of resistance from African leaders.
Ethiopia is an interesting example - since it wasn't colonized in the 1800's, it didn't have colonial rulers who tried to end the practice, so it was one of, if not the, last polities on the continent to end slavery. The League of Nations kept pressuring it to end the institution, and it kept dragging its feet. Slavery was only abolished after Italy invaded and took over the country (Italy used the existence of slavery in Ethiopia as one of the pretexts for the invasion).
This argument is generally used by people to defend bad institutions. I've heard Confederate apologists makes the exact same argument you have here ("Look at the Union treatment of Native Americans. All sides in the Civil War were immoral.").
There are gradations of bad behavior. Treating bad behavior as if it's all the same just encourages a race to the bottom.