This is one of those great perspective questions. I comment here not on your situation with your professor, or on angus's assertion, but in general, because this is an assertion that libertarians make a lot, isn't it? I can see why a libertarian might ignore the entire weight of historical evidence to posit a relationship. The libertarian sees everything from the lens of the singular variable that libertarian-ism deems important. The orange and the pear may be different fruits, but they are both un-banana-like, and in that sense very similar to one another. They are similar in their un-banana-ism. After all, the stars in the constellations look close to each other.
This is basically what I was alluding towards (that and the historical ignorance of those who would like to posit such claims). It's not that I have a problem with libertarianism; it's more that I have a problem with their analytical process that is so far removed from mine and that oftentimes is nonsensical. The way they view policy quandaries makes me want to bash my head against the wall.