This sounds like pro-Bambu wishful thinking rather than an informed opinion. On what basis do you believe proprietary plugins or libraries are allowed under any conditions by the AGPLv3? And if you want to counter that you didn't say "under any conditions", I agree, but that means we need to discuss the particulars of this case to decide whether it's allowed or not.

Bambu took existing open-source, AGPL slicer software for free from the rest of the community and then has continually snubbed that open community by not giving back, or only giving back when they can maintain ultimate control to later decide to be less friendly to their users.

Sorry, no, they can build their own slicer from scratch (hah, good luck) or play by the license. Their networking plugin is tightly integrated with the AGPLv3 Bambu Studio. GNU's stance on plugins is fairly straightforward, and this is not at all a borderline case where the plugin interface is limited: https://www.gnu.org/licenses/gpl-faq.en.html#GPLPlugins