Yeah, the repo is that one. It's only the kernel because it is basically the code available for XD: I don't have code for u-boot used in XD, and I had to use the minibootloader that came with stock. It uses mini version so it also needs uboot,img. That was my main problem trying to update to a newer android version. There is u-boot code for RK3288 in github, with newer kernel versions that should be used for android 6 or 7 at least, but without the config for XD, flashing an incorrect bootloader is dangerous, can be reverted using mask rom, but not something all people know how to do.
If you like a complete SDK, I basically used the firefly RK3288 repository for kitkat. They also have a repo for Lollipop, which ran terrible in firefly for gaming. But without a bootloader then everything is a mess.
There is also an experimental android 7.1 build for Xd which I never tried, and was made by PHH. He aparently used some leaked SDK from a RK3288 tablet, but never leaked that source, so I could never investigate further, so maybe you can try contact him and see if he can leak it to you somehow.