My guess is that you are being modded down because your statement is of a form that many people have heard in real life and associate with poor performance. It shows a lack of thinking about jobs. However, I'm going to give you the benefit of the doubt and assume that you are young and not really worrying about such things.

Basically your answer is like the classic "I could have got a 5.0, but I partied", which is a statement that says a number of things. 1. "I think all the kids who got 5.0 are boring and don't party" (untrue). 2. "I am not capable of getting a 5.0 and partying" (probably true).

That the person would even make the statement says something about their general level of intelligence. Hence when you say "If I had but known, I would have been in the top 100", you are saying "I had no idea that a hugely publicized course at a renowned university might lead to job opportunities". This lack of intelligence seems to contradict your first statement. Or it could be that you are not in the habit of thinking about such things. I hope you are either a kid, or an incredibly wealthy trust-fund baby living in isolation. Usually, however, I hear such remarks from people who are not smart enough to realize they are not smart enough, or they are quitters, or they are victims.