I haven't read the article, but he is basically saying the output transistors could handle that much power if you provided a big enough transformer (and big enough bridge rectifier, and big enough filter capacitor).
Another point that puzzles me is how these diagrams often suggest you can get (say) 30 V DC from a 24 V RMS transformer. At low output currents that may be the case, but if your transformer provides 24 V AC at 10 A, then the most you could possibly get at the output is 24 V DC at 10 A. In reality various power losses and inefficiencies are going to get in the way, so you might expect 24 V DC at less than 10 A, or less than 24 V DC at 10 A. But not both at the same time.