I doubt keltranis is missing the issue. Instead let's assume reddit's devs know what's what think of a why.
My guess is they want to fast bury spam. Spam in old posts will have been deleted and thus old posts are more trustworthy. Voting brigades are a smaller risk than the constant flood of spam.
I believe they have other, closed source, anti-spam measures. This might be the reason, but if it is 'tis a silly reason. If I understand correctly, it means that if 75% of people like a post, and 25% dislike it, that means it has a 25% chance of not being successful because the first vote is negative. Overall, this measure is effectively lowering the quality of all content on the site.
No, spam doesn't have anything to do with it. And actually, older (high-scoring, anyway) links are more likely to have spam comments because the moderators aren't hanging around there anymore.
Maybe you misread, the quote shows it is the intended behavior. They don't care about the number of votes, only if it is more up, equal, or more down. After that they care about age more. The article wants them to base it more on number of votes. You can tell from the parts about returning in whatever order the DB wants that they are going for speed and scalability as well. So more down voted posts being a certain way may not even matter to them as long as they are off the page.
My guess is they want to fast bury spam. Spam in old posts will have been deleted and thus old posts are more trustworthy. Voting brigades are a smaller risk than the constant flood of spam.