Did you read the article? Here's the relevant parts:

  The most likely culprit, it emerged, was that the "collective" and "petty" forms of corruption, as witnessed by Dr Borcan herself, had a curious effect: they might be paid for chiefly by well-off students bribing invigilators, but everyone benefited. It gave the poorer students "a free ride" to higher marks.

  It also meant that when cheating was removed, the academic advantages of wealthier students became even more apparent. Cheating it seemed had provided a kind of levelling effect.

The more correct explanation would seem to be that wealthier students have other advantages - probably along the lines of test preparation, lower stress if it's anything as in the States - which allowed them to retain high performance once cheating was reduced.

I'm not sure from where you're deriving these narratives and counter narratives from.