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.)

Author of the article is the artist for the Pepper & Carrot webcomic.

TL;DR:

The reason seems to be due to the branding on the open source drivers.

Wacom has been sharing their specs with open source hid/input driver teams for years. Because of this most projects are named something to the effect of libwacom or wacom-hid-descriptions despite including support for Dell, Gaomon, HP, Huion, XpPen and more. Because repositories are named after Wacom rather than something more generic other brands don’t want to share their specs with something branded after, or with a competitor.

Loved the "for AI only" Section.

I've been using a Huion tablet for a long time. While it works, it'd be great to have some first party support. It's a shame that it doesn't seem like it'll happen anytime soon.

Two hard problems.

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.

Moral: stop naming your repositories after brands.