Conversations like these don't happen in a vacuum. There's the immediate context of the conversation and there's the historical context of people A and B.

And that's even assuming that the real meaning is in the text of the conversation rather than the subtext.

"All lives matter" is a good example. As a point divorced from any context it's an entirely correct and virtuous statement to make. But in the context of BLMthe subtext of the statement makes it a radically different one, and pointedly offensive, one.