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 consider

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.