He said something like “Okay - of course black lives matter. All lives matter. Only an idiot would say that black lives DON'T matter. So - we all agree that ‘black lives matter’. But that's not what they mean. Using that phrase they are saying that black lives matter, but America has systemic racism, and whites are privileged - so we will force you to make black lives matter. The phrase is code for America and whites are evil. Just try it. Reply to them “Of course black lives matter. But America is not systemically racist. You'll just get shouted at. Or worse.”
Yes, that’s exactly what they mean and that’s a very good reply. :-)