The terms of service are just Discord being transparent about what they’re doing. The thing actually stopping you from using Discord with a third-party client

The terms of service are just Discord being transparent about what they’re doing. The thing actually stopping you from using Discord with a third-party client is that Discord’s server software responds differently to the way the third-party clients talk to it. You’re in charge of what runs on your computer, and they're in charge of what runs on theirs.