Comment by rhyssullivan1

It's from this section of the terms of service:

https://discord.com/developers/docs/policies-and-agreements/...

> Subject to your compliance with the Terms, we grant you a limited, non-exclusive, non-sublicensable, non-transferable, non-assignable, revocable license to access and use the APIs and Documentation we make available to you solely as necessary to integrate with, develop, and operate your Application

When you post on Discord, you grant them a transferable license to your content and that's one of the ways they use it

Disclaimer that it's probably more complicated than that and I'm a software engineer not a lawyer