these are American museums right? telling the story of America. what do non-Americans doing things to non-Americans in not-America have to do with America?
they cover the part that we were responsible for. there are plenty of "fun facts" about slavery in this nation that aren't covered, my favorite is that New Jersey was the last to ban it, in 1866.
The percentage is pretty irrelevant, I have never seen them claim some other percentage owned slaves. so its not like they are lying. and I am not sure what difference 1% makes? is there some magical percentage where Trump would accept it? slaves were owned here.
the first "legal" slave owner bit is a bit of misnomer. Anthony Johnson was the first one to be taken to trial over owning someone for their lives, vs 7 years of indentured servitude. There were other people under similar "indentured servitude for life". Johnson is considered the first because he is the first one to have it challenged and win in court. The Spanish also had outright slavery long before the 1600s in the US. and there were criminal cases where the punishment was lifetime of "indentured servitude".