That article is discussing several different legal actions. I understood Spotify's comments regarding the artists to be in reference to the class action lawsuit

That article is discussing several different legal actions. I understood Spotify's comments regarding the artists to be in reference to the class action lawsuite Wixen is (in their opinion) interfering in. In general to have standing you need to show you have been directly harmed. The exclusive licensee of a song would be directly harmed if someone distributed it without paying for a license. So while Wixen may not have standing in a class-action lawsuit filed on behalf of the original artisits, they would have standing in their own action which they have now filed.