That's actually completely disingenuous, though. It's not like people were going around saying "all lives matter!" and then black people started yelling that black lives matter more. Black people started saying their lives matter and white people somehow took offense to that, and coined this disingenuous phrase as a "sit down and shut up" rebuttal to their assertion that black lives matter.
"Of course black lives matter. Duh. Nobody is disputing that statement." Great. Then why argue with them? Why contend with anything at all? Why not say "Of course! Yes. How can we help?"