I have not read the study but I think it is worth investigating whether they used DNA testing to determine zygosity or whether it was determined from chorionicity (as was typically used prior to DNA testing being readily accessible for research purposes) . Importantly, the understanding now that 25% of dichorionic twins are indeed identical has cast doubt over a lot of previous twin research. Added to which there is now the knowledge of a third type of twin called semi identical or polar body twins.
FWIW I have twins who I assume are fraternal but have not been DNA tested with identical iq scores.
