It's not a true in-universe account but simply the imagination of a version of the Joker that knows that Batman is Bruce Wayne and about Dick Grayson being Robin as well as a lot of the other secrets of the DC Universe.
A clue for this is that this version of Batman is a almost perfect replica of what the Joker wants Batman to be like, in many versions, and it makes sense that he sees the other superheroes as dicks too.
The only part that is potentially but not necessarily real is the appearance of the Joker himself. Think a Joker sitting in Arkham would imagine such things for fun to pass the time, when he's extremely bored. And he seemed to be extremely bored and miserable in the comic, when he appeared...
A clue for this is that this version of Batman is a almost perfect replica of what the Joker wants Batman to be like, in many versions, and it makes sense that he sees the other superheroes as dicks too.
The only part that is potentially but not necessarily real is the appearance of the Joker himself. Think a Joker sitting in Arkham would imagine such things for fun to pass the time, when he's extremely bored. And he seemed to be extremely bored and miserable in the comic, when he appeared...
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