The react/display-name documentation doesn't explicitly state it, but the rule seems to accept display names as properties inside components.

=> Documentation needs to be updated

When the displayName was defined as a property, I think the rule should ensure that the declaration is static, so the following case should be rejected:

class Demo extends Component {
  displayName = "Demo"
}

Whereas the following should be accepted:

class Demo extends Component {
  static displayName = "Demo"
}