You could argue differences in the history of slavery, sure.
Wasn’t creepy or scary though. It was out of character for this group. The racist faces (that’s not an bad-hurt-insult word for you to feels bad – those are literally actually simply racist portrayals) didn’t fit here.
And, you could see why by the responses; did a discussion ensue about slavery? No. It was about the tacky racist meme.
A 1000 better ways to bring it up and you picked that.
If it’s an important topic for you, bring it up another way.
I think you tested the limits of your powers and hit a brick wall here. So, lesson learned.