I've never said such thing. It doesn't matter if you can access things directly: proprietary means it's not FOSS, that's it.
> 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
You need an account to browse Discord, which they can take away from you anytime. If you lose your account you lose your membership to any invite-only communities you belong to, which can be a big deal if you don't have other means to communicate with them to get invited back.
Also, Discord isn't indexable by search engines. So, no there's a big difference between Discord and most web forums.
Also, I don't have anything in particular against Discord (it's miles ahead of Slack in UX for instance, which I hate with a passion), but when people advertise themselves as fans of the old web, and link to their Discord, one can only smirk from the irony.