Well it is the author's expectation, isn't it? I also left the article wondering what in the world conker was. I don't think it matters where the game is from or where the reader lives. It's just good writing to do a bit of explaining for the foreign readers, of which The Guardian knows they have.
I have no clue about american football rules - I barely know the sport exists, the ball has a funny shape, and that it's popular in the states. Still it would be absolute nonsense to explain their basics every time someone talks about them and I won't expect anyone to do so. Explanations like that will just annoy the vast majority of readers, who will have a preexisting interest in the sport and thus will already be familiar with the rules.
That said, I had even less of a clue what the hell conkers is. Luckily I have basic technological literacy and was able to find the wikipedia article[1] in a fraction of the time it takes one to complain.
I guess your point makes sense, though. There isn't the obligation to explain anything. I just personally think it's good writing to anticipate what your readers may not likely understand.