In many of these cities, these DAs are propped up with insane campeign funds, get elected, and refuse to charge, or push for no bail, so someone gets arrested for a felony, they end up back on the street the next day. Just read some of the cases where some people are arrested over and over and keep reoffending before their trial date.
It's a bit of a leap from there to conclude that attempted murder would not be prosecuted.
Does it say anywhere on that memo that attempted murder would not be prosecuted? Of course not. Do these policies generally increase the feeling of lawlessness in the jurisdiction? Yes. Might that precipitate higher degrees of lawlessness? You be the judge.
The OP's argument is likely that the police didn't arrest an attacker because they knew that the DA wouldn't do anything. There is the plausible alternate explanation that the police were engaged in a bad-faith work stoppage or slow down. The third explanation--nonunique to DA--is that the police were overworked or incompetent.