The tradition you quoted on salat al-Layl is authentic.
The meaning of the tradition is deeper. The word shia here doesn’t mean ordinary Shias. It means the first ranking Shias. Such as the position obtained by Maitham al-Tammar, Habib ibn mazhar etc.
Shias are different level. The lowest level is to believe Imam Ali and Ahlul bait are the legitimate successors of the prophet.