Not really a "foss bro" issue, more an issue of privacy guys not understanding software-freedom as its own concern separate from privacy concerns. Same with all the "third party clients" that are just wrappers/mods for the first party client.

There are actual third party discord clients, I use one myself. And, while discord is certainly a terrible service that we should try to lessen our dependency on, it's unfortunate that we have friends and communities on that service and we can certainly use discord with a free client while we make the transition. For me, I use an IRC bridge wherever possible, but bridges generally need to be set up by the "server" "owner" so it's not always an option.

Reading comprehension might be a general problem on reddit and reddit-offshoots (which lemmy certainly is) as I see fans of proprietary software going into free software communities to recommend their favorite proprietary apps. It's certainly not limited to "foss bros."