Basic methods used
If Amazon knows AWS Key length and structure it is prety easy for them to debug WHERE the stored keys are in the app. It really does not require a genius to find those. I could most likely do it, even I do not know the AWS Key length or structure. It just takes a bit longer to hack, as I have to determine where those keys are located.
No coder in his/her right mind is storing anything in plaintext (or hex or bin) within the program files. You might do it in early 90's (as they did), but not in this millenia. As it will be hacked in seconds by using any visual hex/text editor application. It was done 20 years ago, so why not now.