The heavens opened
is a phrase used in Acts 10:11 to describe divine revelation (Acts 10:11) and it seems to have the same intention here. This description of the Son of Man standing at the right hand of God is a revelation granted by God and not an inference made by Stephen or the apostles.
56 Et il dit: Voici, je vois les cieux ouverts, et le Fils de l'homme debout à la droite de Dieu.