Long story short:
theflatearthsociety.org is the original site, from which we split off many years ago (2015 if memory serves). Our theories have since diverged substantially, despite sharing a common ancestor, so to speak.
I can't speak for them, but I believe they were planning to delete their wiki as something they no longer stand by. We try to keep ours up to date, with mixed results.
There is some overlap in membership - many people post on both sites, often under different aliases; but I would argue that the two groups identify as firmly separate, with distinct ideas, beliefs, and approaches.
If you need more info, feel free to shout here, but please keep in mind that we can only offer one side of the story.