I beg to differ, for two possible reasons. Firstly, what is a parsec? Google tells me that a parsec is equal to about 3.26 light years. What's a light year? That's the distance light travels in a year, or about 10 trillion kilometers. Wait. The distance that light travels in a year. Isn't a year a measure of time? So if a parsec measures distance according to time (which it does), then it's perfectly reasonable for Han to use it like he did. He later tells Luke that the Falcon can travel ".5 past lightspeed."
So, people who like to calculate, if the Falcon makes the Kessel Run in 12 parsecs, traveling at a speed that is 1.5 times the speed of light, how many kilometers is the Kessel Run? (Seriously, somebody who knows physics, please check me on this one. Does that work?)
But, for those of you who don't like math and would prefer a more tl;dr explanation with wacky visuals, then try this explanation:
Ever use mapquest or google maps to plot a trip? Did you ever notice how it usually gives you two options when it calculates directions? One is something like "shortest distance," and the other is "shortest time." For guys like Han, who are glorified space truckers, it's going to be very important to know every possible shortcut that exists to get anywhere, both to save on energy expenditure, and to avoid capture (since he's a smuggling space trucker). Now, I hear the argument rising, "But space is huge and mostly empty, and he's got a hyperdrive. How can there be shortcuts in space?" Or something like that. I have an answer to that one, too.
I don't know that it's ever been firmly established what kind of physics is involved in Star Wars, but we know that regular space is all bent and twisted from the gravity wells of planets and stars and things, making the universe analogous to a timey-wimey, wibbly-wobbly ball of yarn floating in hyperspace. Jumping from Tatooine to Alderaan, for example, means that instead of following the thread through all the twists and windings of normal space, the Falcon leaves the thread, shoots through the empty spaces to where Alderaan is, and re-enters the thread. But have you ever looked at ball of yarn? It's all folded up on itself and packed tight, so the Falcon would still have to navigate around the thread of normal space - while still inside the ball of the universe - following the pockets of hyperspace to the right point. Quicker, yes. But still requiring some fancy maneuvering.
Looking at it that way, Han's ability to make the Kessel Run in only 12 parsecs could simply mean that the Kessel Run involves a very convoluted region of normal space where the hyperspace corridors are narrow or difficult to find because of how tightly normal space is all folded up on itself. Han either managed to find a way to navigate around it all in hyperspace that only cost him 12 parsecs, or maybe he managed to re-enter the thread at a point only 12 parsecs from where he jumped, where most pilots (who aren't reckless and crazy) skip the whole region because it's such tricky navigation. Choose your favorite flavor.