There is a huge difference between a hacked-up and partially proprietary OS somehow based on OpenWrt on one hand and on the other having all the drivers needed and a recent Linux kernel built from source, having documented the code and it being accepted by the community after review.
The former is impossible to maintain in the long-run, meaning you will have to live with bugs and vulnerabilities because you can't do anything about it (as not all components are available in source-code form -- and even if we had those sources, I bet the code quality is so low that it would still need a huge amount of work to get it cleaned up and built on top of an up-to-date Linux Kernel).
We may need to put more effort into enforcing that, but it's actually quite clearly stated in our Trademark Policy:
You can use OpenWrt trademarks to make true factual statements about OpenWrt or communicate compatibility with your product truthfully.
So using the name OpenWrt claiming that our firmware is supporting a specific device is only legal if that's actually true. Meaning that the device is supported by the sourcecode on git.openwrt.org and binaries provided at downloads.openwrt.org. If you don't find it there, it's not supported. To make it easier, you can use the OpenWrt Firmware Selector to quickly search among the supported devices.
Going back to your original question: Up to now GL-iNet always chose chips which were already well-supported by OpenWrt for their router products (e.g. QCA or MediaTek). SiFlower SoC is not supported at all yet, neither by OpenWrt nor upstream by the Linux Kernel. So it takes more than just copying over a device-tree file and adding a few lines of shell-code to setup things in userspace...