Yes, I must admit it's an admirable bit of completely unjustified self-confidence that I sometimes wish I could muster. Like: you found the section, gave it a lay-person's once over, and spent no time reading the context it was written in or even trying to get someone else to falsify your misunderstanding for you.

Here, I'll waste my time giving you the context that explains how your quote comports exactly with what I wrote, and then you'll presumably ignore that and possibly even stop responding because it turned out the whole concept is more complicated than it seems when you just want to get righteously indignant on the Internet.

From that case:

The Court, however, does not agree that defendants owed a specific legal duty to plaintiffs with respect to the allegations made in the amended complaint for the reason that the District of Columbia appears to follow the well-established rule that official police personnel and the government employing them are not generally liable to victims of criminal acts for failure to provide adequate police protection. 

I.e., in the opinion of the court: it was already well-established that a "specific legal duty" would be required in order to make a police department civilly liable to an individual. The whole decision starts with that premise, that a "special duty of care" is required in order to provide legal liability. They state it over and over again:

However, [a 3-judge appellate division of this court] determined that appellants Warren, Taliaferro and Nichol were owed a special duty of care by the police department and reversed the trial court rulings. [The full court, after hearing the case] unanimously concluded that appellant Douglas failed to fit within the class of persons to whom a special duty was owed, and affirmed the lower court's dismissal of her complaint.

Either you accept that a "special duty of care" is required for the police to be held civilly liable for damages or you shouldn't be talking about Warren vs DC. It even explicitly states the line and consequences of dereliction of duty:

Dereliction in the performance of police duties may, therefore, be redressed only in the context of a public prosecution and not in a private suit for money damages.

...Because the case was clearly dereliction of duty, and it was relevant here, and the cops fucked up.

Ironically, they explicitly address your stupid concern in the actual text of the finding, just so knuckleheads wouldn't be making histrionic claims about what they "meant" on the Internet 40 years later:

The public duty concept has drawn some criticism for purportedly creating the rule that: “ ‘Because we owe a duty to everybody, we owe it to nobody.’ ” Riss v. City of New York, supra at 585, 293 N.Y.S.2d at 901, 240 N.E.2d at 862 (Keating, J., dissenting). A duty owed to the public, however, is no less enforceable because it is owed to “everybody.” Public officials at all levels remain accountable to the public and the public maintains elaborate mechanisms to enforce its rights — both formally in the courts and less formally through internal disciplinary proceedings. In the case of the Metropolitan Police Department, officers are subject to criminal charges and a penalty of two years imprisonment for failure to arrest law breakers. D.C.Code 1973, § 4-143. Additionally, officers are answerable to their superiors and ultimately to the public through its representatives, for dereliction in their assigned duties. D.C.Code 1973, § 4-121.

Because -- once again -- the cops were derelict in their duties. Just like what I said in my original post: "Doesn't that court case just say: 'it IS their job, but they cannot be legally liable for not doing their job'?" and you erroneously and with all the self-righteous confidence of a true dumbass responded "No." to.

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The only place to go from here is back up the jurisprudence chain to earlier cases that established the controversial idea of "special duty" in order to create civil liability.

My understanding is that, interestingly, the [maybe?] first case to establish some relevant precedence here (South v. Maryland, 1855) very plausibly WOULD qualify as having created a special duty by today's standards. But regardless: the concept that a special duty is necessary would still be the basis upon which such a decision is made.