Because if different educations is the explanation for IQ differences, then that means that without external differences, the twins will have basically the same IQ. That makes it seem like, as long as you control other variables, twin IQ matches - what could cause that other than the fact they have the same genetics, now that other factors have been controlled away?

But like you said, this could also be interpreted as indicating that education is the "sole" or primary determiner of IQ. This is what I mean by the right way to interpret these results being unclear.