It sounds like what you thought you were getting was synchronization, where BackupFolder is copied verbatim to a corresponding folder on GDrive. That’s useful for some things but isn’t a good strategy for backup. If you synchronize the folder on Monday, delete a file on Tuesday, synchronize again on Wednesday and decide on Thursday that the file you deleted was important and you need it back, it will be gone from the backup copy with no way to recover it.
Duplicacy (and other backup software) stores the files and information about them so you can go back to any point in time where you have a backup and recover from a loss. In the scenario outlined above, Wednesday’s backup wouldn’t have the file in it (because it was gone by then), but Monday’s would and you could restore from that. What you found in your GDrive folder is Duplicacy’s way of doing that. I’ll spare you the technical details and say that what Gilbert has done with it (a) is very clever and (b) works very well.