As long as what is going on here is basically comparable to what is going on when a company uses a third-party service as a peer to individuals, then yes, the company probably isn't somehow responsible for what the service is doing. Government Twitter pages have been found to legally constitute public forums, but that was in the context of restricting the government from blocking people. The person whose page it is still don't really run the place and probably isn't responsible for the actions of the platform.
But if a company hires another company to build and operate a communication platform for it (more of a Mailchimp or Invision Community situation), then you probably have a data controller-data processor style relationship.
So, is Discord more like Spotify or is it more like Mailchimp?