I think the point is the thought - in some areas and in some circles - to when a member of intelligentsia says "I have a start-up" is bewilderment: hence the "why didn't you go to college?" follow up question.
The assumption is that if starting/running businesses isn't what people with an education do, unless they have no other choice. The "default" thing for them is to climb the corporate ladder.
pg talked about this in his "high-res society": http://www.paulgraham.com/highres.html
In a way I could see where this idea comes from: if you lack a college degree (or have one but not from a top-tier school and not with top grades) you're still locked out of some opportunities. The only way to a high ranking position in some companies is to enter as a "college hire" and advanced through the ranks (senior positions are seldom open to outside applicants and there is fear that an experienced applicant will be bored/unmotivated at a junior or entry-level position; academic record is seen as the "best" indication of potential success for a junior applicant). So for those without a college education, often the only way to a senior position is through start-ups.
What doesn't follow - any more - is that for those with an education the only way to a senior position is through a big company. It's still a way (there are lots of smart people at places such as Google, IBM, Amazon, Yahoo), but no longer the only reliable option. Even those at big companies no longer stay with the same big company: "job-hopping" is pretty common for advancement (sometimes for better, sometimes for worse - seeing less competent employees but with higher salaries due to greater negotiating leverage - can be de-motivating for employees who are uninterested in job hopping themselves).
The only way that this assumption is still slightly true is that only big companies offer the chance to work on certain classes of problems that start-ups can not tackle - but here's hope that this will change (and there are indication it is changing).