I'm going to call BS here. Folders DO exist as object in Drive, they're technically just specialized files with a mime type of application/vnd.google-apps.folder. Every file in Drive has parent(s) - which are either a folder or the drive root. It is very easy to walk the folder object hierarchy listing what files were in each folder and report on the sizes. Just using the public REST API I could write a script to do this in a few minutes, it's not going to be the fastest thing in the word, but it's not hard to implement. There's no way Google couldn't easily do this.
There's a weak argument to be made that since files can technically belong to several folders that telling you how much data is in one folder and you added all your folder sizes together that they wouldn't match your total drive usage (your total usage may be smaller).