There are limits on how old stars can be. They obviously can't be older than the universe itself. It also took time for star formation to be possible. The paper

There are limits on how old stars can be. They obviously can't be older than the universe itself. It also took time for star formation to be possible. The paper itself argues these are among the oldest stars that can exist in the galaxy, although it doesn't say they are actually the "oldest", just members of the group.