How would 3rd knew Edo Tensei if it was obviously some crazy experiment of Oro's?

Sarutobi had no idea what's going on with hokaes and had to use his exceptional intelligence to get a hold of situation.

He did not know how to stop them at first and was obviously checking the ground with exploding tags with hope that it would be enough.

And, as hokage and anime character, he could sense what's inside frickin' coffins coming from the ground

edit: he could cancel the technique in a similar fashion noob Sakura did with genjutsu used at the chuunin's exam, just on a way other lvl.