"Class ... must implement all abstract methods" inspection

I'm getting the message "Class ... must implement all abstract methods" in a mixin that descends from an abstract class. What I don't understand is why, since the mixin is not instantiated on its own. In the classes that use the mixin, the abstract methods are overridden and made concrete.

The code works fine, but I don't understand the point of the message. Is Pycharm expecting me to signal that this is an abstract class somehow?