You should specify what device it is exactly if you're hoping for people to dig for it. It's probably out there on GitHub under some codenames or part names, sometimes the repo is named after the SoC itself and supports multiple devices that are near identical.

Most of it is going to be just generic LineageOS or CyanogenMod sources with the kernel and some blobs and some libraries. Android's build system is very modular so a single device may pull dozens of repos for various things. You need to track down those repos and with a bit of luck some configs on how to assemble things together.

That website is not liked by the LineageOS community and is often full of crap. But the ROM probably works fine. Maybe you can at least give it a try to see if it's even worth using. It could be someone's barely working proof of concept build for all you know, and even if you do find the sources, it'll suck and require a ton more work. At the very least you can use it to figure out what version of things are used and narrow down your search. If it's built on say, march 17 2015, then that narrows down your search on things around that date.

XDA Forums is also a pretty good place to start your search. If a ROM has been posted publicly, there should be a thread and there should be the sources linked. If not, you can always try messaging the author and see if they still have some stuff around or point you to some docs and sources.

Good luck, compiling Android is a major pain in the ass.