Castle Rock v. Gonzales, 545 U.S. 748 (2005) does not protect a police officer's failure to act.
In Castle Rock, the plaintiff alleged that the police violated the Due Process Clause by failing to enforce a protection order against her husband, who went on to murder their children. The Tenth Circuit held that Gonzales had a property interest in the enforcement of the order that was protected under the Due Process Clause. But the Supreme Court reversed, holding that because state law left police with discretion as to when and how to enforce protection orders, it conferred no property interest that could be protected by the Due Process Clause:
This result reflects our continuing reluctance to treat the Fourteenth Amendment as a font of tort law, but it does not mean States are powerless to provide victims with personally enforceable remedies. Although the framers of the Fourteenth Amendment and the Civil Rights Act ... did not create a system by which police departments are generally held financially accountable for crimes that better policing might have prevented, the people of Colorado are free to craft such a system under state law.
So Castle Rock says that an officer who fails to act is generally going to be safe from a lawsuit that premises his liability on the Due Process Clause. It does not say that officers are immune from consequences for neglecting their duties.
If we can agree that even after Castle Rock, a police officer can be disciplined for sleeping in his car rather than responding to calls for help, the question is not whether an officer can face consequences for failing to act, but rather under what circumstances he can face consequences.
Castle Rock just says that those consequences aren't a due-process issue, any more than they are a separation-of-powers issue or an Establishment Clause issue. So while a due-process lawsuit must fail in such a case, that doesn't mean that a police officer can't face discipline, criminal charges, or a lawsuit based on the violation of some other law.
As the Supreme Court noted, governments are free to write laws creating liability for failing to act, and that was what happened to the officers who murdered George Floyd. Both Minnesota and the United States had written laws that make failing to intervene a criminal offense in certain situations, and the officers were charged under those laws. Castle Rock provides no defense to such a charge.