Hi,
I haven't tried this mod on my own so all I can do is trying to help you with what I have read about it (I'm very interested in this topic as well).
It seems that there are two different versions of the Flashback 8 gold (and the Genesis HD as well). One version can be modified in the way you tried: create a "Games" folder with the bin roms and modify the "all-games.ini" Here's a nice guide for it: https://www.instructables.com/id/Adding-Games-to-Atgames-Genesis-Flashback-HD/
The other version uses .obb files for storage of the games. If you can find an "Android" folder on the device, after it was recognized by your PC as a MTP-device, you do have the obb version. AFAIK the other version won't have this folder.
In this case you'll have to edit the .obb to add more games (Games folder on the SD won't work). Here's a great guide how to do this for the AtGames Genesis: http://atariage.com/forums/topic/266499-new-genesis-flashback/?p=3895318
Last but not least you can simply activate 20 more pre-installed games on some devices by removing the "#" signs from the entries in the all-games.ini file. Maybe these are the bonus games of the Flashback 8 Activision version.
Hopefully that helps a bit. It would be great if someone who managed to do the hack could write a guide for it
Good luck!