The author is making a brilliant argument for getting a secondhand workstation and shoving under their desk.
If you are doing multi machine batch style processing, then you won't be using ondemand, you'd use the spot pricing. The missing argument in that part is storage costs. Managing a high speed, highly available synchronous file system that can do a sustained 50gb/sec is hard bloody work (no S3 isnt a good fit, too much management overhead)
Don't get me wrong AWS _is_ expensive if you are using a machine for more than a month or two.
however if you are doing highly parallel stuff, Batch and lustre on demand is pretty ace.
If you are doing a multi-year project, then real steel is where its at. Assuming you have factored in hosting, storage and admin costs.