The company's reason is "brand protection":

We carefully reviewed the project you shared with us (https://github.com/linuxwacom/wacom-hid-descriptors). While we appreciate the initiative, we found that this is primarily a Wacom-led project, and the potential impact for GAOMON would be quite limited. Even if we added support for our devices, the system would still show the device as a GAOMON model, but the overall setup would display Wacom branding. More importantly, participating would require sharing our device specifications directly with Wacom – which is not something we can consider.

The last part of that reply is very wrong.

The article also has a reply from Peter Hutterer, a "senior software engineer at Red Hat and a maintainer of Linux's core input device handling infrastructure since decades", which is worth reading.

I'm feeling pretty justified in sticking with Wacom. I only replaced my first tablet because it had a 4:3 aspect ratio and a serial port. (I could've lived with the latter, but the aspect ratio was making things skewed on a modern display.)