At three-thirty that afternoon, Harry, Ron, and the other Gryffindors hurried down the front steps onto the grounds for their first flying lesson. It was a clear, breezy day, and the grass rippled under their feet as they marched down the sloping lawns toward a smooth, flat lawn on the opposite side of the grounds to the forbidden forest, whose trees were swaying darkly in the distance.
The Slytherins were already there, and so were twenty broomsticks lying in neat lines on the ground. Harry had heard Fred and George Weasley complain about the school brooms, saying that some of them started to vibrate if you flew too high, or always flew slightly to the left.
––– Harry Potter and the Sorcerer's Stone
There’s subject-auxiliary inversion by the pronoun for locative complement, so. (1) In here, is so re-placing already there? (2) If the previous locative complement were there not already there, still could the inversion be proper (The Slytherins were there, and so were~) ?