It looks like they're at least pretty close to 19.07, so the question is how much 'sauce' they have added. It looks like the hardware is using common SoCs (ath79, mt7628, ...) so the hardest part (SoC support) is certainly already done. Things to look at would be how their master branch looks, since anything you'd like to upstream to OpenWrt proper will need to go to master first.
This wiki page has a bit more detail on adding a new device, might help you to find what bits and code is worth looking into from the Kroks repo:
This branch is 1181 commits ahead, 7884 commits behind openwrt:master.
They are based on 19.07, so possible related changes to master since the 19.07 branching should be taken care of.
The easiest might be to port just the few commits on top of their branch that actually define their device.
However, it looks like they do some stuff somewhat strangely/brute-force-like (e.g. modify usb device database, modify global boot scripts by adding separate functions instead of just adding their devices etc.). They have not really looked for upstreaming capability while adding the support for their devices.
so, it would require some adaptation work in order to fit into the main OpenWrt.
It would still be up to someone with the device on their desk to integrate this (which involves fixing up quite a bit) and to get it working, this can't be done blindly but requires basic development skills and the ability to build- and test the changes safely (meaning with the opened device and a serial console attached).