Jones says “Black lives matter.”
Smith replies “All lives matter” in a not-so-subtle rebuke. Apparently Smith was interpreting Jones as saying “Only black lives matter.”
Such a response suggests to me that Smith has a tin ear. Jones is surely not saying “Only black lives matter.” Rather, Jones is saying “Black lives matter too.” Or, more fully, “Black lives matter just as much as other lives.”
If you want to see why it is so important to say that “Black lives matter,” you could start with the fact that people choose to misunderstand when you say it.