It looks more like they had all the jupyter notebooks for all customers sitting side by side, and each notebook had a master key for the related DB.

...but due to a poor deployment strategy, the instances were not sandboxed from each other.

(and yes, you would be correct in thinking that was a catastrophic failure; it's like having a folder with a directory per customer and a master access key in each folder, and WOOOPS, didn't realize people might look at ../Amazon/master_key.txt

It's pretty hard to brush this one off; it's a colossal screw up)