I'm not a ROM developer and I'm going off memory, too lazy to read up on it. If I'm getting this completely wrong, someone feel free to correct it.
From what I remember, LOS uses its own version of the kernel. I assume that device specific changes, taken from the device kernel sources, are merged into the LOS kernel. In this case the Moto G3 kernel changes will have been merged in long ago and are sufficient to be used with all versions of LOS since.
In our case, with the sources available, we cannot get a good enough FOD implementation. The latest MIUI kernel used with LOS gives a good FOD experience.
To be official LOS all hardware should work to the same level as stock ROMs, so we cannot get certified with a greatly inferior FOD functionality. Therefore we need the latest kernel sources, to get the FOD changes in the LOS kernel.
Again, I'm not a ROM developer and so this is going off memory and from me piecing together bits of info I've picked up from different places, so it might not be completely correct.
---------- Post added at 01:34 PM ---------- Previous post was at 01:32 PM ----------
The LOS requirements state that hardware should work as well as stock ROM, so we cannot get official with what's availableIt would be possible to use the outdated kernel sources to make LineageOS official for this phone but it would also offer a much poorer user experience than this unofficial ROM with the prebuilt kernel. But If the maintainer of this ROM thinks it is not worth the effort it would not happen.