By your reasoning, any backend that’s not accessible directly by the user is proprietary. HN would then also be proprietary, because it relies on a third party (HN) and uses a nonstandard closed protocol (this comment section). Any website that stores data and exposes it via their own UI is proprietary (I leave this vague on purpose, as it is up to interpretation). Discord can be run in the browser and is accessible via APIs. I think it’s as open as any other web app by a company that has a commercial interest.
Services like Discord have lowered the entry barrier for non-technical conversation and community-building online. It certainly seems to be polarizing. I’ve noticed a lot of my friends and colleagues either embrace it or despise it. I wouldn’t mind an open-source alternative to it, but with the extensive features that it offers that’s hard to achieve.