That is not a good enough excuse considering you are obliged to publish the source code under the GPLv2 license.
Edit: And also, any source code you do publish must also be able to be built with just the published files, and function correctly, as per the GPLv2.
I want to help you guys with helping the community. I have worked with several OEMs in the past, committed to helping open source. Feel free to PM me here, or message me on Telegram (@amyosx)
I'm sorry but I can't tell you that yet. I think that's not a problem to share the source codes of older models but newer models need further analysis to avoid security risks.
The kernel source released is not complete. There are lots of missing folders/files for example include/ which contain all the headers. Please release the complete kernel-4.14 folder. There is also vendor/mediatek/kernel_modules which contains the mediatek kernel module. Those 2 folders are all umidigi needs to release. Everything else is your private code.