Sure, it's a different story if you have a server farm and need a client farm to test it. But in his scenario you could use an off-the-shelf laptop to saturate the local link with requests...

Even if it's not easy to tell the min/max bounds in high scale deployments, it's possible to give an approximate value by measuring elements. You have load balancers that don't do more than X req/s, servers that take X ms/req, databases which can do X q/s, caches with delay X ms in X% of request. That gives you an rough estimate - then it's about locating bottlenecks which prevent you from reaching those numbers. (simplified model)

Amazon EC2 is a great place to get of load simulators.

Another challenge is the difference between volume of traffic, and diversity of traffic. Naively set up load tests tend to be really good tests of your caching mechanism, and non-tests of other parts of your system.

At high scale, you should have a few grand to pay gomez for some real-world load testing without having to buy your own hardware. Also, it sounds like the author did not even try a "workstation with jmeter". I am by no means a scalability expert but even I know better than to stress-test on a live system.