Semantics seems to be the issue here, and you’re not being totally honest when you claim Mrs Obama’s statement to be correct, then later qualify your position. It’s not difficult to discern the difference between building something and being a part of building something. I would assume Mrs Obama is intelligent enough to know the difference also, yet didn’t say it. It’s simply much more dramatic and poignant to leave the impression of slaves exclusively building the structure. You may disagree and I assume you will, but you will be wrong in doing so, because that is the bare bones of the issue.