Well, if it were proved to be true that S's are actually less likely to have graduate degrees, the other way of looking at that might be that the higher education system needs to rethink its approach to be a better fit for different learning styles.
I think it was Voltaire who disdained the "modern" obsession with teaching everything by lectures. He mocked the educational system by saying that he didn't understand why everything had to be taught from books and in organised classes, and called for more acknowledgement of independent study and learning, and practical, hands-on methods of learning.
I think what he might've been getting at, in MBTI terms, is that maybe more S's would have graduate degrees, if they were given a chance to learn and practice their subjects the way they prefer, rather than doing the N thing of abstracting it all most of the time.
My own experience of S's - especially ISTP's - is that they're all too happy to learn and have plenty of curiosity. I've often given out some bit of pointless trivia, expecting it to be met with 'so what?', and in fact it's met with "wow, that's cool, why's that then?' in a room full of Sensors.
I also stand utterly in awe of many Sensing craftsmen and women that I know, who are able to produce a beautifully crafted item from a block of wood within a matter of hours, which is perfectly functional. Whenever I try to do it, I'm so impatient and so willing to cut corners, so oblivious to details, that all the times I say 'ahh, close enough' during the project, take their toll on the final product! That's a kind of intelligence that doesn't get enough recognition, I think.
But I think education is unfair to Sensors in much the same way that socialising is unfair to introverts. In socialising, it's an E's world, so the E's write the rules, often disenfranchising the I's, as it were. Higher education seems to have evolved to become an N's world, with N's writing the rules and deciding the criteria, thereby disenfranchising the S's.