No, it's that the message of "black lives matter" is intended as a reminder to a society that acts like it has forgotten this or doesn't give it enough consideration. "All lives matter" as a rebuttal to it is a deliberately-obtuse "gotcha". "Well yeah but all lives matter, so there, you should use that slogan instead of making it about race". Everyone got that already. What (apparently) needed to be pointed out was a more specific thing. And most people understand context or they wouldn't be able to communicate, period, and the context of "black lives matter" obviously doesn't mean other lives don't, as anyone who's not trying to play jerkass quibbling games would already recognize. It's a deliberate misunderstanding for the sake of distraction.

Logically if people have forgotten black lives matter then they have forgotten that all lives matter. I think people who get caught up arguing against logic are purposefully being devisive.

In the context of police brutality, white lives don't matter. Who protests when an unarmed white person is killed by police?