“Something like 7%” is noticeably less than twice 4%, so no, I didn’t make that assumption. (I also explicitly stated that this was a back-of-the-napkin-quality analysis.)
It is noticeably closer to 8% than to 4% though. 7% would imply graduates exhibit a rather strong preference for marrying non-graduates over other graduates, which I find hard to believe.
At a time when only 4% of the population had graduate degrees, if 25% of married grad-degree-holders were married to another grad-degree-holder, that would constitute a large preference in favor of other graduates.