projectUsers includes everyone who is, or was, assigned to a task, issue or as the owner, even if they're no longer assigned.
So iterating over that list may not be what you want.
The fact you get the "requestor" sounds like that person either converted the issue to a project, or Fusion - regardless, s/he appears in the People tab of the project.
You could filter what you display by role e.g.
- IFIN({user}.{roleID}, IDofRole1, IDofRole2, CONCAT({user}.{name}," - ",{user}.{role}.{name}), "")
However - note that if you DON'T display a list item you get 2 <hr>s - there's no way to filter the collection you iterate over.