This doesn't quite make sense to me. Or, it makes sense, but not for the reasons presented on the surface.
If you're Andreesen Horowitz, you have access to a nearly-unlimited supply of capital. You also have incredible access to information and dealflow throughout the industry. In situations in which you are not capital constrained, the most efficient way to establish a workforce is to acquire it from somewhere else; that's why acqui-hires make so much sense, because you've basically had someone else do the hard (and often completely unpredictable) work of figuring out if people can work together in a successful manner (which, I should note, is different from determination of whether the thing they're working on is successful). If I were A16Z, I would arbitrage my information advantage to secure a high quality engineering team by acquiring a company that had smart people but a lame product; going through a traditional hiring process would be a waste of my time (and since everyone is throwing money at my firm, it would also be a waste of time that could be better spent on figuring out where to deploy my capital).
A16Z is rich enough to buy people, if it really wants people, for internal use (whether it is for their own company needs, or for some idea that they want to incubate inside of the firm). And even if they don't want to buy a whole set of people, they have enough wisdom and experience to know that you want to hire within a cohort of successful people rather than randoms (to see this in practice, take a look at Don Melton's post about Eazel alumni ending up at Apple on the front page right now). This tells me that their programming test has more to do with some sort of evaluation of this Hacker Rank service ("tell you what, we will run our own test on the service to see what happens!") or some other sort of analytical adventure.
Or, you know, maybe it is really a programming test after all. Maybe they are hoping to find some sort of diamond-in-the-rough engineers who are socially and career-wise disconnected from an existing network. Some sort of "black swan programmer" or something. But I highly doubt it.
So basically, if you go through this test, you're helping A16Z evaluate something; that something, however, probably isn't you.