Lang wasn’t arrested immediately, but he was arrested. From the very article you linked:

During the press conference, Mayor Jacob Frey and MPD announced the arrest of Lang and his co-conspirators, with help from state troopers, after both vehicles were stopped and shortly impounded following the events at City Hall.

The article goes into why he wasn’t arrested at the scene. You can disagree with this decision (I certainly do), however don’t make it sound like Lang and Co got away scot-free.