These overseas qualifications are said to meet the same standards as UK postgraduate qualifications. But the GMC points out that they are not the same as UK qualifications in every way. They also don't guarantee entry to UK specialty training.
I don't think that whether or not these doctors also have to take PLAB exams is a significant part of the problem. The study was concerned with comparison of PLAB and UK graduates' performance on Royal College of Physicians' MRCP(UK) and Royal College of General Practitioners' MRCGP exams.