Other twin studies didn't look solely at twins, but compared them with non-twins. As poster civilized said https://news.ycombinator.com/item?id=31304805:

monozygotic twins reared apart show much more similar IQs than unrelated individuals raised together, indicating that genetics are the dominant driver of IQ. https://www.gwern.net/docs/iq/1996-bouchard-2.pdf