I don't read it like that at all. Both solutions use the Amazon cloud. Only in one solution you distribute a lot of processes, just because it's possible, and easy to code. When they figured out that rampant distribution was costly, they put more thinking in keeping a lot of computation in the same place (so, "monolith", but still in the cloud). No surprise, they found great savings. If they hadn't, they wold not have written about it. But they had to put some (most likely major) effort into redesigning the application.