I can get behind the Sunday Times paywall. Here are two key paragraphs:
*Of the 30 top private schools such as Eton College and Winchester, only one is exclusively doing GCSEs while the rest are largely taking the international GCSE, widely seen as an easier test. More than 500,000 state school pupils are doing the new tougher GCSEs, which Nick Gibb, the schools minister, has said are “more rigorous”.
Head teachers have warned that state pupils could end up with worse exam results, which would “shut them out from top universities through no fault of their own”, according to Richard Cairns, headmaster of Brighton College. Unlike all his private school rivals, Brighton has chosen to do only GCSEs this summer.*
The rest of the story contains not a shred of evidence to show whether iGCSEs are or are not easer. No study, no stats, nothing. It's even ambiguous as to whether Nick Gibb has called the new GCSEs more rigorous than iGCSEs or just more rigorous than old GCSEs!
It reads as though Richard Cairns is the sole source for this article and he is not a disinterested party.
I'm not remotely qualified to make any claims about relative standards, but I would be very surprised if "top" private schools opted for "cushy" exams at 16 because surely they want their students to have the best possible preparation for A levels?