This phenomenon isn't limited to England. The one thing all Harvard undergrads learn (probably as much from one another as from the professors) is how to defend themselves in conversation. Some are smart, and some aren't, but by the time they graduate they all know how to sound smart.
I think the effect is more significant at Oxbridge though. At Oxbridge you have "supervisions" or "tutorials" as an undergrad. These consist of a 1-on-1 lesson with a fellow or grad student where you are questioned mercilessly on every section of your essay or problem set for an hour. They ask you about every single sentence or step in your solutions. You have to prepare carefully, and even then you have to learn to…
However, don't extrapolate this to a 'normal' undergrad's experience at Oxford.
I've never worked as hard in my life as for my MBiochem finals at Oxford - my law conversion is widely depicted as being intense, but didn't compare.
If the author of that article truly believes oxford tutorials are about defence, he's getting shortchanged, or he's looking to them for exactly the same thing our ruling elite gets from them; either way, he's shortchanging his readers in giving voice to such a view. The true aim of a tutorial is not to spend an hour defending what you wrote two nights ago, it's about letting that stuff fall into the backdrop to a cutting edge discussion with a leading (often foreign) academic; not just getting a better picture of what the cutting edge is, but where it's likely to develop to and what could be investigated over the next few years so we can get there. In UK undergraduate education, my understanding is that this focus on developing questions and probing soft spots in knowledge, rather than just assembling and defending answers, is pretty radical. And it certainly seems to run contrary to a fair amount of the article's claims.
Admittedly most of a tutorial's exchange of ideas flows to the student, but I rarely met a tutor who won't admit to having got something out of tutorials they have given, too.