How would that prevent you from forking the app? F-droid isn’t a repository for the code of the app. I don’t think this is related at all.
I don’t actually know the reason why it’s not on F-droid but I assume it has some historical reason. It has never been on F-drroid since Text-secure. Moxy Marlinspike was strictly against it afaik. If somebody has more detail on it, feel free to share it.