That's not actually what it stands for...

ACAB & ACAC both span multiple english speaking countries, predate BLM days by at least three or four decades, and are hardly idiotic in the face of documented abuses of power by rotten apples, coupled with tacit acceptance of the bad ones by their wider fraternity.

Interestingly, it isn't strictly "leftist". In fact it was used by both racist and anti-racist skinheads.