I'll spell it out for you:
It suggests that it needs to be said because there are lots of people who don't know it. However, everybody knows black lives matter, and it's an implication that everybody who doesn't adopt your race ideology is a racist. In reality, those who do are actually racists in denial and are projecting. I and a lot of other people feel this way.
Then, there are the people who subscribe to your ideology, who think that white people are racist and need to be told to stop being racist, and there are a fair number of people who believe that.
So, there are a lot of people who think one thing, and a lot of people who think another thing. Shouting your shibboleths in the street that declares your difference highlights the division that exists in society. That is what we call "divisive."
Oh, and since this is the first time I see you in this thread, I must say I'm sorry I wasn't able to divine that you were waiting for an explanation.