The Scrum Guide (SG) explicitly explains that those who participate in DS are developers. The word participation is very important so that it can be defined that, apart from the developer, other people may join/listen as long as they don't participate. What about the Product Owner (PO) and Scrum Master (SM)? they may attend and are only allowed to participate if they are part of the development team. For example: it often happens that a person who is a Business Analyst (BA) also doubles as SM. In this case, the person's participation is as BA when speaking on DS, not as SM.