Just launched it in VM. I see a linux kernel, lightdm, Gnome utilities, systemd and some usual open source software, like hexchat, vlcplayer, etc. So they wrote a DE (budgie) using GTK and Python, and probably installer and package manager.
Edit: what I mean is they didn't "write the OS". They built a distribution (and yes, it's still a lot of work if done properly), but hardly "from scratch".