This is slang from the UK although it is used in Australia as well and can be used in number ways:
1 To make a foolish statement;
2 To talk nonsense or rubbish;
3 To say something which is quite obviously false;
4 To exaggerate your abilities or knowledge of a subject;
5 To bluff or boast about something;
6 To be a self-aggrandising twat;
“Malcolm just tried out a total load of bollocks on me; I told him straight, you’retalking through your arse.”
A term to indicate someone's questioning of the validity of one aspect in a fictional story is in fact overshadowed by the absurdity of the story's basic premise.
A phrase first coined when one person loudly wondered how the ducks in a Donald Duck story managed to breathe under water without any visible breathing apparatus on their diving suit, and another replied "You know you're reading a story about talking ducks, right?"
Seb: "So how did they hide a physical object in a mirror's reflection in the first Harry Potter?"