Your argument rests on the assumption that the serial comma is demonstrably preferable, that it is foolish not to use it in all cases. It is possible that the authors of this advice are better placed to asses whether their audience is comfortable with nuance and context. It is how Australians learn to read and write.

Well, the serial comma is demonstrably preferable, full stop, regardless of your appeal to Australian Authority.

But putting that aside, the issue is not whether the Oxford Comma is preferred: it's what this website is actually instructing bureaucrats to do.