I only got to know about LibrePCB at the end of 2016 after several months of development on Horizon EDA. While there are some similarities such as the use of UUIDs and strong link between schematic and netlist, there are major architectural differences such as Horizon EDA using raw OpenGL vs QTSceneGraph(?) and Gtkmm vs QT.

As far as I can tell, the development models also differ significantly: The LibrePCB devs seem to be focused on doing things the right way the first time such as having a proper operation-based undo/redo stack whereas horizon just stores a copy of the whole schematic/board for undo/redo. They also seem to focus more on simplicity for beginners rather than more advanced features such as length tuning and advanced design rules.

Putting development effort where I like enabled me do to the board for my master thesis https://github.com/carrotIndustries/x-band-tx/ (4 Layers, differential pairs, length-matched traces) in Horizon EDA only after about 2 years after the initial beginning of development.