Owen Jones is a young left-wing writer from the UK. He describes himself as a fourth generation socialist. His grandfather was a member of the Communist Party and his parents were Trotskyists.
So what happens when you grow up in such a socialist family? Well, for one thing you don't call your mum "mum":
That tweet was made in the context of Owen Jones describing his mother's commitment to feminism.
It's that idea again that we should be self-defining and therefore we shouldn't be defined by a traditional or inherited or natural role like that of mother. Therefore, Owen is not allowed to call his mother "mum".
It's a logical application of the theory, but it suggests that the theory itself is misguided and that self-definition is not always and everywhere the principle to be followed.
Hat tip: David Thompson