Then they re-engineered it to scale.
If they started not with a simple RoR prototype but with a ton of scalable infrastructure, they'd launch maybe a couple years later, and burn much, much more money before the launch.
If at all; probably obvious, but important to point out that in a startup environment this kind of mistake could just outright kill the company.
I work on an open source project (Sandstorm[1]) that was started by a now-defunct startup. There's a repo that's unmaintained at this point (It might still work?) containing a high-scalability backend that had been used to manage Sandstorm Inc's hosted offering. Apparently they never got to the scale where they couldn't have thrown one giant server at it and been fine, and it pulled a lot of resources away from other (ultimately more important) development tasks.
...and now I'm stuck developing against mongo for no good reason :P
[1]: https://sandstorm.io