No architecture scales infinitely, you merely shift at each step what the next bottleneck will be. Even with small scale sites you'll see this: first, you need to move from everything on one server to a load balancer plus app servers talking to a single database. Then you need to share your database. Then you need to move to multiple load balancers. Then so on and so forth, depending on the specifics of your service, mixed in with various rounds of local optimization at the application level that buy you just a little more time before having to ship another architectural change.
Most distributed systems don't really scale ass you add nodes beyond a factor of 10 to 100. If they did, you wouldn't need to hire super expensive people to handle those systems.
The cloud enables easier scaling, it's not a silver bullet. No matter what, scaling plans have a region where they break down, so it doesn't imply they aren't designed for good scaling.
Are they not using cloud, which should enable easy scaling? Or is their architecture not designed for good scaling?