If you were curious like I was, about why this fork was necessary, I found this on their About page:
KeePassXC is a community fork of KeePassX which aims to incorporate stalled pull requests, features, and bug fixes that have never made it into the main KeePassX repository.
Judging from TFA it's due to KeePassX having one maintainer, so the pull requests all bottleneck with him. Also mentioned is that one desirable outcome of this is that KeePassXC developers are given co-maintainer status of KeePassX and that they re-merge down the line.
KeePassXC is a community fork of KeePassX which aims to incorporate stalled pull requests, features, and bug fixes that have never made it into the main KeePassX repository.