Although the word "unlimited" gets thrown around in casual usage, of course no company is going to pay you forever if you never show up for a day of work. You've still got to get your job done at a high level perspective (say quarterly) but the idea is that management is not tracking every hour you show up to work or decide to take off; they're looking at results not attendance.
In that light I'm reading this as essentially setting the expectations bar very low, potentially to nothing, for a year.
Basically this policy and pretty much all of Netflix's policies come down to behaving like a professional and treating other people with the same expectations. If you lose that culture and either side starts to misbehave of course things could go very badly. But that culture is very important to Netflix and they are always working consciously to preserve it.
I haven't seen any official stats about vacation (as someone else pointed out, in order to do that they'd have to track it) but I've never known anyone to have a problem with it. I've seen quite a few people take 3 weeks or so at a time because they're going overseas, and in Silicon Valley that's pretty unheard of.
Disclaimer: I'm not a lawyer, I don't speak for the company, yada, yada. I'm just a rank and file employee interpreting what I know.