It's not obscure in the UK. You can find any number of equally 'obscure' cultural practices referenced in American newspapers without explanation. It's unreasonable to expect a British newspaper to be free of British cultural background assumptions just because it's viewable by an international audience.

It appears that about 38% of visits to their site come from the UK, so it might be that the majority of their readers aren't familiar with conker. https://www.similarweb.com/website/theguardian.com/#geograph...

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.

The NYT has 150 million monthly readers globally of which 27% are international. I still wouldn't expect it to insert helpful explainers every time it mentions a cultural phenomenon specific to the US.