I'd say the core of their success is running everything in a single rack in a single datacenter at first (for months? a year?) and getting lucky. Life is simple when you don't need the costs and effort of reliability upfront.
In any case, not everyone need five nines, and usually it's just much easier to bring down a platform due to some bug in your own software rather that the core infrastructure going down at a rack level.
It's probably the main reason why they were able to get away with this and why their application does not need scalability. I see they themselves are only offering two 9s of uptime.