They're using HalfShot's appservice bridge I think, which works entirely through the standard Discord API with bot users. IIRC Discord is very much aware of the project.
Discord's generally fine with anything using it's API/gateways as long as it's NOT logging in as a "real" user.
There's two ways to connect discord. One is a appservice. You got a bot user that relays everything and you get messages that are basically "BotUser: <Alice> Hello world" using the official API. This is currently allowed, but doesn't allow access to 1:1 chats. The other is "puppeting", the bot logs in as Alice by Alice providing her credentials and hitting endpoints the actual Web app uses. This results in a less obvious, nicer experience for people on he discord side but is the one that is banned
Discord's generally fine with anything using it's API/gateways as long as it's NOT logging in as a "real" user.