I don't even consider companies stupid enough to put out a blurb like that.
To extend the car analogy, it's more like a lifetime warranty on a car: the car might break down all the time, but its still covered.
But seriously speaking, a 99.99% SLA is not more realistic or achievable in itself than any other SLA promise. It all depends on the provider (how they strive to reach that promise) and what they promise in return if the promise is broken.
They're explicitly not saying that it will never go down. They're saying that if it ever does go down, they will pay their customers a penalty. Without having any "acceptable" amount of downtime that they don't have to pay a penalty for.
Really, too-many-9s SLAs are what's ridiculous. So you say you "guarantee" 99.99% uptime, but only interruptions of 30min or more actually count? 2 incidents of 29 minutes would put you over that .001% that you claim, without either counting.