Dog whistle politics has changed. In the past, a "dog whistle" was when a politician said something that his supporters would know was racist, but centrists wouldn't hear it. Only supporters, only the target demographic that the speaker intended to mobilize, heard the dog whistle.
Today, slogans like "All Lives Matter" are dog whistles intended not to be heard by the core audience. Instead of being heard by the voters that a politician is trying to mobilize, they are heard by opponents. Only your opponents hear the dog whistle — which gets your opponents dump their anger on your target demographic. People saying "All Lives Matter" feel good about themselves, about not seeing race. And progressives get angry at them. There's no faster way to drive them to circle their wagons around Trump than to have progressives yelling at them. That is the point: that is why a politician trying to win the next election keeps bringing up little bits of xenophobic and racist garbage — he is campaigning, he is setting traps, we are falling for those traps.
Social media makes this particularly effective: if someone is unhappy about something in their lives, shares a grumpy post that casually tosses some grumpiness about that problem, and then faces holy hell from progressives — it drives them to the person who put the dog whistle in their hands and encouraged them to blow it.
It used to be that politicians blew dog whistles: now they use social media to encourage their followers to blow the dog whistles, without being able to hear them.