There's a slave ancestor in everyone's past if you go back far enough, I'm sure.
The first black people on these shores arrived as slaves, in Virginia, within a decade or so of the Jamestown settlement. I haven't heard of any blacks arriving here from Africa as free men, pre-Civil War, so unless his grandparents came through Ellis Island or something, he is the descendant of slaves.
But to what Denzel said--no, HE's not an ex-slave, he's a descendant of slaves. But being a descendant of slaves is absolutely nothing on the "I have suffered" scale compared to an actual slave.
Anyhow, I don't know what he meant or was trying to say, since I didn't see the clip, but I haven't seen him around making an ass out of himself like many other Hollywood types. He's an extremely talented actor, and it may be he wants to keep his true political views to himself--which is fine by me. Would that everybody in Hollywood would do the same.
The vast majority of blacks are descendants of slaves, and Mr. Washington is probably among them, but there were free black men who colonized America, albeit not many. There were also black slaveholders who were quite wealthy and who owned more slaves than many of the white slaveholders. The following is a source for this interesting piece of American history.
http://www.americancivilwar.com/authors/black_slaveowners.htm