Maybe it's easyer to understand if you think that a Joker can break any rules. If there is no battenburg sign and you know there is Joker in that 2x2 area, then it doesn't matter what the other digits are.
this is not completely correct. At least half of the battenburg should be respected. In the example, it is obvious that the Battenburg is either 3 or 5 from the top corner. Hovever, R2C1 and R2C2 should still be one even number and one odd number, with the even number located below the joker.