It seems that that particular fork of the Assets Bundle Browser only supports UnityFS up to 2019.2.*
I've no idea if any of the ABB forks are being actively developed. Here's the list (No hits in the first 100 listed):
https://github.com/Unity-Technologies/AssetBundles-Browser/network/members
** This is actively developed and works on the .bundle files I tested with:
https://github.com/Perfare/AssetStudio/releases/tag/v0.16.21