Its been a while since I looked further into it, but the individual components were being built with buildstream and I think they were looking to switching the image building to mkosi with buildstream integration (from a quick look at the gnome-build-meta repo it looks like it still didn't happen).

Also a minor nitpick: I wouldn't call Flatpak a/their "dependency manager". It is more like an application manager, closer to winget than a traditional package manager.