Sorry, but if I'm reading the article correctly you didn't save any money, because you weren't using AWS in the first place. All you did was to do some back of the napkin math what it might cost you to run something on AWS.
Isn't it how you save money by picking cheaper product instead of more expensive?
We save by buying hardware and renting DC space.
If we were using AWS we would not save.
predictions are hazy, hindsight is 20/20. There's a reason there are dozens of companies dedicated to cutting cloud costs because surprise, they aren't configured well. Most of these probably severely overestimate compute needs and don't factor things like autoscaling.